drm/sti: sti_vtg: Handle return NULL error from devm_ioremap_nocache

Here, If devm_ioremap_nocache will fail. It will return NULL.
Kernel can run into a NULL-pointer dereference. This error check
will avoid NULL pointer dereference.
Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
Acked-by: Vincent Abriou <vincent.abriou@st.com>

drivers/gpu/drm/sti/sti_vtg.c

@@ -429,6 +429,10 @@ static int vtg_probe(struct platform_device *pdev)
 		return -ENOMEM;
 	}
 	vtg->regs = devm_ioremap_nocache(dev, res->start, resource_size(res));
+	if (!vtg->regs) {
+		DRM_ERROR("failed to remap I/O memory\n");
+		return -ENOMEM;
+	}
 
 	np = of_parse_phandle(pdev->dev.of_node, "st,slave", 0);
 	if (np) {