drm/nouveau/kms/nv50: workaround EFI GOP window channel format differences

[ Upstream commit e78b1b54 ]

Should fix some initial modeset failures on (at least) Ampere boards.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>