x86/efi: properly handle 0 in pixel reserved bitmask

x86/efi: properly handle 0 in pixel reserved bitmask

In some graphics modes firmware is allowed to return 0 in pixel reserved
bitmask which doesn't go against UEFI Spec 2.8 (12.9 Graphics Output Protocol).

Without this change non-TrueColor modes won't work which will cause
GOP init to fail - observed while trying to boot EFI Xen with Cirrus VGA.

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