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: c4d8cfe5 ("drm/msm/dsi: add implementation for helper functions")
Cc: Sibi Sankar <sibis@codeaurora.org>
Cc: Sean Paul <seanpaul@chromium.org>
Cc: Rob Clark <robdclark@chromium.org>
Cc: <stable@vger.kernel.org> # v4.19+
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Rob Clark <robdclark@chromium.org>

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

@@ -421,15 +421,15 @@ static int dsi_clk_init(struct msm_dsi_host *msm_host)
 	}
 
 	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;
 	}