gma500: Final enables for Oaktrail

This switches the ifdef to match the Kconfig so that Oaktrail probing occurs
and adds some additional minor bulletproofing.

Tested on a Fujtisu Stylistic Q550 internal display. HDMI might work but that
remains to be seen.

Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>