drm/i915: Only start with the fake-irq timer if interrupts are dead

As a backup to waiting on a user-interrupt from the GPU, we use a heavy
and frequent timer to wake up the waiting process should we detect an
inconsistency whilst waiting. After seeing a "missed interrupt", the
next time we wait, we restart the heavy timer. This patch is more
reluctant to restart the timer and will only do so if we have not see any
interrupts since when we started the fake irq timer. If we are seeing
interrupts, then the waiters are being woken normally and we had an
incoherency that caused to miss last time - that is unlikely to reoccur
and so taking the risk of stalling again seems pragmatic.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170217151304.16665-5-chris@chris-wilson.co.uk