drm/gma500: remove useless comment

Remove useless comment in framebuffer.c.

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/1476165825-12137-1-git-send-email-jiang.biao2@zte.com.cn