For the off-chance we have an interrupt posted and haven't processed the
CSB.

v2: Include tasklet enable/disable state for good measure.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20180326115044.2505-4-chris@chris-wilson.co.ukReviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>