drm/msm/dsi: Fix return value check for clk_get_parent

clk_get_parent returns an error pointer upon failure, not NULL. So the
checks as they exist won't catch a failure. This patch changes the
checks and the return values to properly handle an error pointer.

Fixes: c4d8cfe516dc ("drm/msm/dsi: add implementation for helper functions")
Cc: Sibi Sankar <[e-mail]>
Cc: Sean Paul <[e-mail]>
Cc: Rob Clark <[e-mail]>
Cc: <[e-mail]> # v4.19+
Signed-off-by: Sean Paul <[e-mail]>
Signed-off-by: Rob Clark <[e-mail]>

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

···
 	}
 
 	msm_host->byte_clk_src = clk_get_parent(msm_host->byte_clk);
-	if (!msm_host->byte_clk_src) {
-		ret = -ENODEV;
+	if (IS_ERR(msm_host->byte_clk_src)) {
+		ret = PTR_ERR(msm_host->byte_clk_src);
 		pr_err("%s: can't find byte_clk clock. ret=%d\n", __func__, ret);
 		goto exit;
 	}
 
 	msm_host->pixel_clk_src = clk_get_parent(msm_host->pixel_clk);
-	if (!msm_host->pixel_clk_src) {
-		ret = -ENODEV;
+	if (IS_ERR(msm_host->pixel_clk_src)) {
+		ret = PTR_ERR(msm_host->pixel_clk_src);
 		pr_err("%s: can't find pixel_clk clock. ret=%d\n", __func__, ret);
 		goto exit;
 	}