drm/i915: change dev_priv->fbc.plane to dev_priv->fbc.crtc

drm/i915: change dev_priv->fbc.plane to dev_priv->fbc.crtc

Since the mapping from CRTCs to planes is fixed, looking at the CRTC
is essentially the same as looking at the plane. Also, the next
patches wil start using the frontbuffer_bits macros, and they take the
pipe as the parameter instead of the plane, and this could differ on
gens 2 and 3.

Another nice thing is that we don't risk accidentally initializing
things to PLANE_A if we don't set the value before it is used for the
first time. But this shouldn't be a problem with the current code.

V2: Rebase.

Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com> (v1)
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>