drm/i915: Stop dropping irq around resets

A long time ago, we were afraid of handling interrupts and signaling
waiters during a reset, worrying that the confusion in request handling
would interfere with our attempts to process the reset in an orderly
fashion. Since then, we have isolated our irq-driven request handling by
virtue of the engine->timeline.lock and control of kthreads where
required, eliminating the danger of concurrently processing interrupts.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180806145647.13131-1-chris@chris-wilson.co.uk