drm/i915/chv: Add cursor pipe offsets

Unsurprisingly the cursor C regiters are also at a weird offset on CHV.
Add more pipe offsets to handle them.

This also gets rid of most of the differences between the i9xx vs. ivb
cursor code. We can unify the remaining code as well, but I'll leave
that for another patch.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Antti Koskipää <antti.koskipaa@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>