drm/i915: call assert_device_not_suspended at gen6_force_wake_work

Because we shouldn't be runtime suspended when forcewake is supposed
to be enabled.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
[danvet: Update commit message - no WARN expected since the bugfix for
issues hit with this assert is already in. And resolve conflicts with
the change from worker to timer for the delayed fw release.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>