modetest: call drmModeFreePlaneResources

24 (16 direct, 8 indirect) bytes in 1 blocks are definitely lost in loss record 2 of 7
   at 0x402994D: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
   by 0x4A25950: drmMalloc (xf86drm.c:147)
   by 0x4A2E26D: drmModeGetPlaneResources (xf86drmMode.c:951)
   by 0x4025FF: dump_planes (modetest.c:276)
   by 0x4052AF: main (modetest.c:1120)

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>