drm/i915/gen9: Make power well disabling synchronous

So far we only power well enabling was synchronous not disabling. Since
we don't exactly know how the firmware (both DMC and PCU) synchronizes
against the actual power well state during DC transitions, make the
disabling also synchronous.

CC: Mika Kuoppala <mika.kuoppala@linux.intel.com>
CC: Patrik Jakobsson <patrik.jakobsson@linux.intel.com>
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/1459515767-29228-6-git-send-email-imre.deak@intel.com