drm/i915: Don't set queue_priority_hint if we don't kick the submission

If we change the priority of the active context, then it has no impact
on the decision of whether to preempt the active context -- we don't
preempt the context with itself. In this situation, we elide the tasklet
rescheduling and should *not* be marking up the queue_priority_hint as
that may mask a later submission where we decide we don't have to kick
the tasklet as a higher priority submission is pending (spoiler alert,
it was not).

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: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191021080226.537-1-chris@chris-wilson.co.uk