drm/i915: Cancel pending execlist tasklet upon wedging

drm/i915: Cancel pending execlist tasklet upon wedging

Highly unlikely, but if the stop_machine() did suspend the tasklet, we
want to make sure that when it wakes it finds there is nothing to do.
Otherwise, it will loudly complain that the ELSP port tracking no longer
matches the hardware, and we will be mightly confused.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170621124804.4529-1-chris@chris-wilson.co.uk
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>