drm/i915: avoid struct mutex output_poll mutex lock loop on unload

Cancel the output polling work proc before acquiring the struct mutex
to avoid acquiring the work proc mutex with the struct mutex
held. This avoids inverting the lock order seen when the work proc
runs.

Signed-off-by: Keith Packard <keithp@keithp.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>