drm/i915/gen9: flush DMC fw loading work during system suspend

Currently during system s/r we enable/disable DC6, so before we do so
make sure that the firmware loading is complete.

Note that whether we need to enable DC6 for S3/S4 is still open.  At
least the firmware program is lost during S3 and we need to reprogram it
after resuming. Until this is clarified we keep the current behavior and
enable/disable DC6.

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Animesh Manna <animesh.manna@intel.com>
Tested-by: Daniel Stone <daniels@collabora.com> # SKL
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1446069547-24760-13-git-send-email-imre.deak@intel.com