drm: mxsfb: drm_dev_alloc() returns error pointers

We should be checking for IS_ERR() instead of NULL because
drm_dev_alloc() returns error pointers.

Fixes: 45d59d704080 ("drm: Add new driver for MXSFB controller")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20161213122332.GA7519@elgon.mountain

drivers/gpu/drm/mxsfb/mxsfb_drv.c

@@ -395,8 +395,8 @@ static int mxsfb_probe(struct platform_device *pdev)
 		pdev->id_entry = of_id->data;
 
 	drm = drm_dev_alloc(&mxsfb_driver, &pdev->dev);
-	if (!drm)
-		return -ENOMEM;
+	if (IS_ERR(drm))
+		return PTR_ERR(drm);
 
 	ret = mxsfb_load(drm, 0);
 	if (ret)