drm/i915: Disable engine->irq_tasklet around resets

When we restart the engines, and we have active requests, a request on
the first engine may complete and queue a request to the second engine
before we try to restart the second engine. That queueing of the
request may race with the engine to restart, and so may corrupt the
current state. Disabling the engine->irq_tasklet prevents the two paths
from writing into ELSP simultaneously (and modifyin the execlists_port[]
at the same time).

Fixes: 821ed7df ("drm/i915: Update reset path to fix incomplete requests")
Testcase: igt/gem_exec_fence/await-hang
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170208143033.11651-3-chris@chris-wilson.co.uk