drm/i915/bxt: Suspend power domains during suspend-to-idle

drm/i915/bxt: Suspend power domains during suspend-to-idle

On SKL/KBL suspend-to-idle (aka freeze/s0ix) is performed with DMC
firmware assistance where the target display power state is DC6. On
Broxton on the other hand we don't use the firmware for this, but rely
instead on a manual DC9 flow. For this we have to uninitialize the
display following the BSpec display uninit sequence, just as during
S3/S4, so make sure we follow this sequence.

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/1459515767-29228-8-git-send-email-imre.deak@intel.com