drm/msm/dsi: Fix return value check in msm_dsi_host_set_display_mode()

In case of error, the function drm_mode_duplicate() returns NULL
pointer not ERR_PTR(). The IS_ERR() test in the return value check
should be replaced with NULL test.
Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Reviewed-by: Archit Taneja <architt@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>

drivers/gpu/drm/msm/dsi/dsi_host.c

@@ -2278,9 +2278,9 @@ int msm_dsi_host_set_display_mode(struct mipi_dsi_host *host,
 	}
 
 	msm_host->mode = drm_mode_duplicate(msm_host->dev, mode);
-	if (IS_ERR(msm_host->mode)) {
+	if (!msm_host->mode) {
 		pr_err("%s: cannot duplicate mode\n", __func__);
-		return PTR_ERR(msm_host->mode);
+		return -ENOMEM;
 	}
 
 	return 0;