drm/msm: remove redundant NULL check

Fix below warnings reported by coccicheck:
./drivers/gpu/drm/msm/msm_gem.c:991:3-9: WARNING: NULL check before some
freeing functions is not needed.

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Jiapeng Zhong <abaci-bugfix@linux.alibaba.com>
Signed-off-by: Rob Clark <robdclark@chromium.org>