drm/gpu: Fix Memory barrier without comment Issue

-Issue found using checkpatch.pl
-Insert comments for memory barrier usage

Signed-off-by: Bhanusree <bhanusreemahesh@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/1571984858-4644-1-git-send-email-bhanusreemahesh@gmail.com