gma500: mark staging broken

It now clashes with upstream DRM which we don't want to block.

We don't want to delete this code just yet as we want to keep it for
comparison and reference when debugging, but soon it will be a removal
candidate as well

Signed-off-by: Alan Cox <alan@linux.intel.com>
Cc: linux-next@vger.kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>