gma500: Fix regression on Oaktrail devices

The register map patches didn't set one value for the GMA600 which
means the Fujitsu Q550 dies on boot with the GMA500 driver enabled.

Add the map entry so we don't read from the device MMIO + 0 by mistake.

Signed-off-by: Alan Cox <alan@linux.intel.com>
Cc: Horses <stable@vger.kernel.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>