gma500: allow non stolen page backed framebuffer

For Moorestown at least we may not have stolen RAM with which to back the
initial framebuffer. Allow a GEM backing.

At this point we should have all the bits in place needed to make it work once
it has been debugged.

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