drm: Propagate error from drm_fb_helper_init().

The previous commit fixes the problem, these commits make sure we actually
fail properly if it happens again.

I've squashed the commits from Chris since they are all fixing one issue.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Dave Airlie <airlied@redhat.com>