drm/i915: Add disp2d power well for chv

Not sure if it's still there since chv has per-pipe power wells.
At least with current Punit this doesn't work. Also the display
irq handling would need to be adjusted for pipe C. So leave the
code iffed out for now.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>