efifb: Implement vga_default_device() (v2)

EFI doesn't typically make use of the legacy VGA ROM, but it may still be
configured to pass that through to a given video device. This may lead to
an inaccurate choice of default video device. Add support to efifb to pick
out the correct active video device.

v2: fix if->ifdef

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