freedreno/kgsl: fix pointer-to-int cast

Fixes this warning:

freedreno/kgsl/kgsl_ringbuffer.c: In function ‘kgsl_ringbuffer_flush’:
freedreno/kgsl/kgsl_ringbuffer.c:149:19: warning: cast from pointer to integer of different size [-Wpointer-to-int-cast]
   req.timestamp = (uint32_t)kgsl_ring->bo->hostptr;
                   ^

Signed-off-by: Eric Engestrom <eric.engestrom@imgtec.com>
Reviewed-by: Rob Clark <robdclark@gmail.com>