drm/i915: Flush the irq and tasklets before asserting engine is idle

Before we assert that the engine is idle, make sure we flush any
residual tasklet. After that point, if the engine is not idle, more work
may be queued despite us trying to park the engine and go to sleep.

References: https://bugs.freedesktop.org/show_bug.cgi?id=103479


Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171101202149.32493-1-chris@chris-wilson.co.uk


Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>