drm/i915/gt: Use proper priority enum instead of 0

I915_PRIORITY_NORMAL is 0 so use that instead for better
readability.

Cc: John Harrison <John.C.Harrison@Intel.com>
Signed-off-by: Nirmoy Das <nirmoy.das@intel.com>
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231023121305.12560-1-nirmoy.das@intel.com

drivers/gpu/drm/i915/gt/intel_engine_heartbeat.c

@@ -188,7 +188,7 @@ static void heartbeat(struct work_struct *wrk)
 			 * low latency and no jitter] the chance to naturally
 			 * complete before being preempted.
 			 */
-			attr.priority = 0;
+			attr.priority = I915_PRIORITY_NORMAL;
 			if (rq->sched.attr.priority >= attr.priority)
 				attr.priority = I915_PRIORITY_HEARTBEAT;
 			if (rq->sched.attr.priority >= attr.priority)