drm/i915: Use __ffs() in for_each_priolist for more compact code

Gcc has a slight preference if we use __ffs() to subtract one from the
index once rather than each use:

__execlists_submission_tasklet              2867    2847     -20

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/20190226102404.29153-11-chris@chris-wilson.co.uk