freedreno: fix potential use-after-free on a5xx+

Something that valgrind spotted:

==8441== Invalid read of size 4
==8441==    at 0x5DEE168: msm_ringbuffer_emit_reloc (msm_ringbuffer.c:506)
==8441==    by 0x5B48F0F: OUT_RELOCW (freedreno_util.h:241)
==8441==    by 0x5B48F0F: fd5_emit_blit (fd5_emit.h:131)
==8441==    by 0x5B48F0F: emit_gmem2mem_surf.isra.12 (fd5_gmem.c:450)
==8441==    by 0x5B4910F: fd5_emit_tile_gmem2mem (fd5_gmem.c:477)
==8441==    by 0x5B14943: render_tiles (freedreno_gmem.c:342)
==8441==    by 0x5B14943: fd_gmem_render_tiles (freedreno_gmem.c:416)
==8441==    by 0x5B0FBA7: batch_flush (freedreno_batch.c:281)
==8441==    by 0x5B0FBA7: fd_batch_flush (freedreno_batch.c:306)
==8441==    by 0x5B11FE7: fd_context_flush (freedreno_context.c:52)
==8441==    by 0x58AD783: st_glFlush (st_cb_flush.c:121)
==8441==    by 0x5751EE7: _mesa_make_current (context.c:1652)
==8441==    by 0x58E6A97: st_api_make_current (st_manager.c:811)
==8441==    by 0x5A2CE43: dri_unbind_context (dri_context.c:207)
==8441==    by 0x5A2C77F: driUnbindContext (dri_util.c:589)
==8441==    by 0x4AC8A67: MakeContextCurrent (glxcurrent.c:214)
==8441==  Address 0x6f5eb1c is 204 bytes inside a block of size 240 free'd
==8441==    at 0x4868F44: realloc (vg_replace_malloc.c:785)
==8441==    by 0x5DEE143: msm_ringbuffer_emit_reloc (msm_ringbuffer.c:502)
==8441==    by 0x5B48F0F: OUT_RELOCW (freedreno_util.h:241)
==8441==    by 0x5B48F0F: fd5_emit_blit (fd5_emit.h:131)
==8441==    by 0x5B48F0F: emit_gmem2mem_surf.isra.12 (fd5_gmem.c:450)
==8441==    by 0x5B4910F: fd5_emit_tile_gmem2mem (fd5_gmem.c:477)
==8441==    by 0x5B14943: render_tiles (freedreno_gmem.c:342)
==8441==    by 0x5B14943: fd_gmem_render_tiles (freedreno_gmem.c:416)
==8441==    by 0x5B0FBA7: batch_flush (freedreno_batch.c:281)
==8441==    by 0x5B0FBA7: fd_batch_flush (freedreno_batch.c:306)
==8441==    by 0x5B11FE7: fd_context_flush (freedreno_context.c:52)
==8441==    by 0x58AD783: st_glFlush (st_cb_flush.c:121)
==8441==    by 0x5751EE7: _mesa_make_current (context.c:1652)
==8441==    by 0x58E6A97: st_api_make_current (st_manager.c:811)
==8441==    by 0x5A2CE43: dri_unbind_context (dri_context.c:207)
==8441==    by 0x5A2C77F: driUnbindContext (dri_util.c:589)
==8441==  Block was alloc'd at
==8441==    at 0x4868F44: realloc (vg_replace_malloc.c:785)
==8441==    by 0x5DEE08B: msm_ringbuffer_emit_reloc (msm_ringbuffer.c:481)
==8441==    by 0x5B48F0F: OUT_RELOCW (freedreno_util.h:241)
==8441==    by 0x5B48F0F: fd5_emit_blit (fd5_emit.h:131)
==8441==    by 0x5B48F0F: emit_gmem2mem_surf.isra.12 (fd5_gmem.c:450)
==8441==    by 0x5B4909F: fd5_emit_tile_gmem2mem (fd5_gmem.c:465)
==8441==    by 0x5B14943: render_tiles (freedreno_gmem.c:342)
==8441==    by 0x5B14943: fd_gmem_render_tiles (freedreno_gmem.c:416)
==8441==    by 0x5B0FBA7: batch_flush (freedreno_batch.c:281)
==8441==    by 0x5B0FBA7: fd_batch_flush (freedreno_batch.c:306)
==8441==    by 0x5B11FE7: fd_context_flush (freedreno_context.c:52)
==8441==    by 0x58AD783: st_glFlush (st_cb_flush.c:121)
==8441==    by 0x5751EE7: _mesa_make_current (context.c:1652)
==8441==    by 0x58E6A97: st_api_make_current (st_manager.c:811)
==8441==    by 0x5A2CE43: dri_unbind_context (dri_context.c:207)
==8441==    by 0x5A2C77F: driUnbindContext (dri_util.c:589)

Signed-off-by: Rob Clark <robclark@freedesktop.org>