drm/i915: Refactor VLV display power well init/deinit

We do the exact same steps around the disp2d/pipe A power well
enable/disable on VLV and CHV. Refactor the shared code into
some helpers.

Note that this means we now call vlv_power_sequencer_reset() before
turning off the power well, whereas before we did it after. That
doesn't matter though since vlv_power_sequencer_reset() just resets
the power sequencer software tracking and doesn't touch the hardware
at all.

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>