gma500: Don't try and take a GEM handle of a non GEM fb

The initial GMA500 framebuffer is not GEM but stolen memory. We can't
therefore take a GEM handle of it. Stop anyone trying to do this and causing
a crash.

Ideally we need a way to have GEM handles to non GEM objects but it's not
clear how and if GEM and the modesetting/fb interfaces it provides are
supposed to or indeed if they can handle it.

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