drm/i915/gen9: Use flush_work to synchronize with dmc loader

During driver unload to ensure we dont have any pending task,
flush_work added to complete firmware loading task.

v1: Initial version.

v2: As per review comments from Daniel,
Removed flush_work from skl_set_power_well. As we have taken
power well refernece and rpm count during firmware loading
by using display_power_domain_get/put - this will always
ensure rpm will be blocked if firmware is not loaded.

Cc: Daniel Vetter <daniel.vetter@intel.com>
Cc: Damien Lespiau <damien.lespiau@intel.com>
Cc: Imre Deak <imre.deak@intel.com>
Cc: Sunil Kamath <sunil.kamath@intel.com>
Signed-off-by: Animesh Manna <animesh.manna@intel.com>
Signed-off-by: Imre Deak <imre.deak@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/1446069547-24760-12-git-send-email-imre.deak@intel.com