drm/i915: Defer kicking the tasklet until all rescheduling is complete

Since we may kick more than engine, and may kick each one a couple of
times, coalesce the tasklet execution to the end. This also ensures that
we have the chance to run the tasklet immediately after priority
bumping.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200331114852.11583-1-chris@chris-wilson.co.uk