drm: fb: cma: Fail gracefully on allocation failure

The drm_gem_cma_create() function never returns NULL but rather an error
encoded in the return value using the ERR_PTR() macro. Callers therefore
need to check for errors using the IS_ERR() macro. This change allows
drivers to handle contiguous DMA allocation failures gracefully.
Signed-off-by: Thierry Reding <thierry.reding@avionic-design.de>
Acked-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/drm_fb_cma_helper.c

@@ -220,7 +220,7 @@ static int drm_fbdev_cma_create(struct drm_fb_helper *helper,
 
 	size = mode_cmd.pitches[0] * mode_cmd.height;
 	obj = drm_gem_cma_create(dev, size);
-	if (!obj)
+	if (IS_ERR(obj))
 		return -ENOMEM;
 
 	fbi = framebuffer_alloc(0, dev->dev);