drm/gma500: add comments for new parameters

Added comments for new parameters.

Signed-off-by: Jiang Biao <jiang.biao2@zte.com.cn>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1476238699-25820-1-git-send-email-jiang.biao2@zte.com.cn