gpu/drm: fix a few kernel-doc "/**" mark warnings

The opening comment mark "/**" is reserved for kernel-doc comments, so
it will generate warnings for comments that are not kernel-doc with
"make W=1". For example,

drivers/gpu/drm/drm_memory.c:2: warning: Cannot understand  * \file
drm_memory.c

Signed-off-by: Qian Cai <cai@lca.pw>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/1563198173-7317-1-git-send-email-cai@lca.pw