drm/i915/execlists: Hesitate before slicing

Be a little more hesitant before injecting a timeslice, and try to take
into account any change in priority that is due for the running task
before switching to another task. This will allow us to arbitrarily
prevent switching away from a request if we deem it necessarily to
disable preemption, for instance.

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>
Cc: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Acked-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190703091726.11690-9-chris@chris-wilson.co.uk