efi_loader: Add DM_VIDEO support

Some systems are starting to shift to support DM_VIDEO which exposes
the frame buffer through a slightly different interface.

This is a poor man's effort to support the dm video interface instead
of the lcd one. We still only support a single display device.

Signed-off-by: Alexander Graf <agraf@suse.de>
[trini: Remove fb_size / fb_base as they were not used]
Signed-off-by: Tom Rini <trini@konsulko.com>