drm/i915: implement crtc helper semantics relied upon by the fb helper

Yikes!

But yeah, we have to do this until someone volunteers to clean up the
fb helper and rid it of its incetious relationship with the crtc
helper code.

Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>