drm/simpledrm: Fix an NULL vs IS_ERR() bug

The devm_memremap() function doesn't return NULL, it returns error
pointers.

Fixes: 9a10c7e6 ("drm/simpledrm: Add support for system memory framebuffers")
Signed-off-by: Dan Carpenter <error27@gmail.com>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/Y9JHzImRcUaa0mi1@kili

drivers/gpu/drm/tiny/simpledrm.c

@@ -719,8 +719,8 @@ static struct simpledrm_device *simpledrm_device_create(struct drm_driver *drv,
 		drm_dbg(dev, "using system memory framebuffer at %pr\n", mem);
 
 		screen_base = devm_memremap(dev->dev, mem->start, resource_size(mem), MEMREMAP_WC);
-		if (!screen_base)
-			return ERR_PTR(-ENOMEM);
+		if (IS_ERR(screen_base))
+			return screen_base;
 
 		iosys_map_set_vaddr(&sdev->screen_base, screen_base);
 	} else {