drm: add kernel doc for exported gem dmabuf_ops

Signed-off-by: Samuel Li <Samuel.Li@amd.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1516311860-24949-1-git-send-email-Samuel.Li@amd.com