efi/boot: make sure graphics mode is set while booting through MB2

efi/boot: make sure graphics mode is set while booting through MB2

If a bootloader is using native driver instead of EFI GOP it might
reset graphics mode to be different from what has been originally set
by firmware. While booting through MB2 Xen either need to parse video
setting passed by MB2 and use them instead of what GOP reports or
reset the mode to synchronise it with firmware - prefer the latter.

Observed while booting Xen using MB2 with EFI GRUB2 compiled with
all possible video drivers where native drivers take priority over firmware.

Signed-off-by: Igor Druzhinin <igor.druzhinin@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Release-acked-by: Juergen Gross <jgross@suse.com>