drm/i915: Define VLV/CHV display power well domains properly

Currently we're using POWER_DOMAIN_MASK as the power domains for the
display power well on VLV/CHV. That includes all power domains even
though the disp2d/pipe-a power well is not needed for a lot of things.
Let's reduce these to what we actually need.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1460977348-32260-3-git-send-email-ville.syrjala@linux.intel.com


Reviewed-by: Imre Deak <imre.deak@intel.com>