drm/i915/skl: re-enable power well support

Now that the known DMC/DC issues are fixed, let's try again and
re-enable the power well support.

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Patrik Jakobsson <patrik.jakobsson@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1447869230-21416-1-git-send-email-imre.deak@intel.com