drm/i915: use intel_crtc for the FBC functions

This is all internal i915.ko work, let's start using intel_crtc for
everything.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>