drm/i915/execlists: Drop unused parameter to lookup_priolist()

lookup_priolist() no longer attaches the request into the priolist, it
just returns the priolist for the given priority instead. Drop the
unused parameter.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180508003046.2633-1-chris@chris-wilson.co.uk