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=103479Signed-off-by: NChris 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.ukReviewed-by: NMika Kuoppala <mika.kuoppala@linux.intel.com>