drm/i915/execlists: Shortcircuit queue_prio() for no internal levels

If there are no internal levels and the user priority-shift is zero, we
can help the compiler eliminate some dead code:

Function                                     old     new   delta
start_timeslice                              169     154     -15
__execlists_submission_tasklet              4696    4659     -37

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200525075347.582-4-chris@chris-wilson.co.uk