drm/i915: Synchronize irq before parking each engine

When we park the engine (upon idling), we kill the irq tasklet. However,
to be sure that it is not restarted by a spurious interrupt after doing so,
flush the interrupt handler before parking. As we only park the engines
when we believe the system is idle, there should not be any interrupts to
distrub us; so flushing the final in-flight interrupt should be sufficient.
(However, we are still dependent on the HW behaving in an orderly and
timely fashion, which we shall endeavour to improve upon later.)

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Cc: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171023213237.26536-2-chris@chris-wilson.co.uk
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>

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