drm/i915: Kill the tasklet then disable

Disabling the tasklet leaves it if scheduled on the ready to run list
until it is re-enabled. This will leave the ksoftird thread spinning
until satisfied. To prevent this situation on starting the GPU reset, we
want to kill the tasklet first and then disable. The same problem will
arise when a tasklet is scheduled from another device, so a better
solution is required for the general case.

Reported-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Fixes: 1f7b847d

 ("drm/i915: Disable engine->irq_tasklet around resets")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170212172002.23072-3-chris@chris-wilson.co.uk
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>