drm/gpu: Add comment for memory barrier

-Add comment for memory barrier
-Issue found using checkpatch.pl

Signed-off-by: Bhanusree <bhanusreemahesh@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/1571891313-14341-1-git-send-email-bhanusreemahesh@gmail.com