gpu: remove gma500 stub driver

In v3.3, the gma500 drm driver moved from staging to drm group by
Alan Cox's 3abcf41f patch. the gma500 drm driver should control
brightness well and don't need gma500 stub driver anymore.

Reference:
http://lists.freedesktop.org/archives/dri-devel/2012-May/023426.html
http://lists.freedesktop.org/archives/dri-devel/2012-May/023467.html



Acked-by: Matthew Garrett <mjg59@srcf.ucam.org>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Lee, Chun-Yi <jlee@suse.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>