drm/i915/guc: Keep request->priority for its lifetime

We also want to support preemption with GuC submission backend.
In order to do that, we need to remember the priority, like we do on
execlists path.

v2: Remove completed prio == INT_MAX optimization
Signed-off-by: NMichał Winiarski <michal.winiarski@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Jeff McGee <jeff.mcgee@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: NChris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: NChris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20171025200020.16636-10-michal.winiarski@intel.com

drivers/gpu/drm/i915/i915_guc_submission.c

@@ -641,7 +641,6 @@ static void i915_guc_dequeue(struct intel_engine_cs *engine)
 			}
 
 			INIT_LIST_HEAD(&rq->priotree.link);
-			rq->priotree.priority = INT_MAX;
 
 			__i915_gem_request_submit(rq);
 			trace_i915_gem_request_in(rq, port_index(port, execlists));