drm/i915/chv: Add unique power well ID for the pipe A power well

drm/i915/chv: Add unique power well ID for the pipe A power well

The power well IDs are used for lookup, so they must be unique on a
given platform; ensure this on CHV. This didn't cause an actual problem
since we didn't need to look up power wells which happened to share an
ID.

Mark this new power well as custom, since its programming pattern
doesn't follow that of the rest of VLV/CHV power wells.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1499352040-8819-2-git-send-email-imre.deak@intel.com
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>