drm/i915: Simplify CHV pipe A power well code

The pipe A power well is the "disp2d" well on CHV and pipe B and C wells
don't even exist. Thereforce we can remove the checks for pipe A vs.
others and just assume it's always pipe A.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Sivakumar Thulasimani <sivakumar.thulasimani@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>