drm/fb: revert the i915 Actually configure untiled displays from master

This code moved in here in master, so revert it the same way.

This is the same revert as 9fa24625

 ("Revert "drm/i915/fbdev:
Actually configure untiled displays"") in drm-fixes.

Signed-off-by: Dave Airlie <airlied@redhat.com>