drm: add parameter explanation for some gem dmabuf_ops

To reduce some warnings.

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Samuel Li <Samuel.Li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1522092145-12645-1-git-send-email-Samuel.Li@amd.com