efi/gop: Add support for BLT_ONLY adapters

efi/gop: Add support for BLT_ONLY adapters

EFI GOP has support for multiple different bitness types of frame buffers
and for a special "BLT only" type which is always defined to be RGBx.

Because grub2 doesn't ever directly access the frame buffer but instead
only renders graphics via the BLT interface anyway, we can easily support
these adapters.

The reason this has come up now is the emerging support for virtio-gpu
in OVMF. That adapter does not have the notion of a memory mapped frame
buffer and thus is BLT only.

Signed-off-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>