x86: Use vga_default_device() when determining whether an fb is primary

IORESOURCE_ROM_SHADOW is not necessarily an indication that the hardware
is the primary device. Add support for using the vgaarb functions and
fall back if nothing's set them.

Signed-off-by: Matthew Garrett <mjg@redhat.com>
Cc: mingo@redhat.com
Acked-by:  <hpa@zytor.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>