drm/i915: Uninit the DMC FW loader state during shutdown

drm/i915: Uninit the DMC FW loader state during shutdown

We need to wait for the DMC FW loader work to complete during shutdown,
even if it's unlikely to be still pending by that time, fix this.

This also fixes the wakeref tracking WARN during shutdown about the
leaked reference we hold due to a missing DMC firmware.

While at it add a TODO comment about unifying the shutdown and PM
power-off sequences and later these sequences with the driver remove and
system/runtime suspend sequences.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
References: https://lore.kernel.org/lkml/20210303055517.GB2708@xsang-OptiPlex-9020
Reported-and-tested-by: kernel test robot <oliver.sang@intel.com>
Reported-and-tested-by: Edward Baker <edward.baker@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210311144529.3059024-1-imre.deak@intel.com