drm/i915: Use the "display core" power domain in vlv/chv set_cdclk()

drm/i915: Use the "display core" power domain in vlv/chv set_cdclk()

The PFI credit programming performed during cdclk change on vlv/chv
requires access to a register in the disp2d power well. So far
we've abused pipe-A power domain for this, but now we have the
more appropriate "display core" domain so let's make use of it.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190701161534.6671-1-ville.syrjala@linux.intel.com
Reviewed-by: Imre Deak <imre.deak@intel.com>