Revert "drm/i915/bxt: Disable power well support"

With the preceding fixes power well support should be functional on
Broxton, I could enter/exit DC5 without problems.

This reverts commit 18024199.

CC: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: David Weinehall <david.weinehall@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1459515767-29228-16-git-send-email-imre.deak@intel.com