drm/i915: Nuke the VLV/CHV PFI programming power domain workaround

The hack to grab the pipe A power domain around VLV/CHV cdclk
programming has surely outlived its usefulness. We should be
holding sufficient power domains during any modeset, so let's
just nuke this hack.

v2: Fix typo in commit message (Ander)

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Ander Conselvan de Oliveira <conselvan2@gmail.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170120182205.8141-13-ville.syrjala@linux.intel.com