drm/i915/execlists: Show the "switch priority hint" in dumps

Show the timeslicing priority hint in engine dumps to aide debugging.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200305135843.2760512-1-chris@chris-wilson.co.uk