drm/msm/mdp: fix a problematic usage of WARN_ON()

WARN_ON() takes a condition rather than a format string. This patch
converted WARN_ON() to WARN() instead.
Signed-off-by: Geliang Tang <geliangtang@163.com>
Signed-off-by: Rob Clark <robdclark@gmail.com>

drivers/gpu/drm/msm/mdp/mdp4/mdp4_kms.h

@@ -157,7 +157,7 @@ static inline uint32_t mixercfg(uint32_t mixer_cfg, int mixer,
 			COND(mixer == 1, MDP4_LAYERMIXER_IN_CFG_PIPE6_MIXER1);
 		break;
 	default:
-		WARN_ON("invalid pipe");
+		WARN(1, "invalid pipe");
 		break;
 	}