drm/i915: Don't cancel DRRS worker synchronously for flush/invalidate

It's not needed since the worker rechecks that it didn't race. We only
need to cancel synchronously after disabling drrs to make sure the
worker really is gone (e.g. for driver unload). But for normal
operation the stall is just wasted time.

Reported-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Ramalingam C <ramalingam.c@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Vandana Kannan <vandana.kannan@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>