efifb: BGRT: Add nobgrt option

In some setups restoring the BGRT logo is undesirable, allow passing
video=efifb:nobgrt on the kernel commandline to disable it.

Reported-by: David Herrmann <dh.herrmann@gmail.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: David Herrmann <dh.herrmann@gmail.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>