drm/vc4: Use devm_drm_dev_alloc

We can simplify a bit the bind code, its error path and unbind by using
the managed devm_drm_dev_alloc function.

Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20201029190104.2181730-5-maxime@cerno.tech

drivers/gpu/drm/vc4/vc4_drv.h

@@ -72,7 +72,7 @@ struct vc4_perfmon {
 };
 
 struct vc4_dev {
-	struct drm_device *dev;
+	struct drm_device base;
 
 	struct vc4_hvs *hvs;
 	struct vc4_v3d *v3d;
@@ -235,7 +235,7 @@ struct vc4_dev {
 static inline struct vc4_dev *
 to_vc4_dev(struct drm_device *dev)
 {
-	return (struct vc4_dev *)dev->dev_private;
+	return container_of(dev, struct vc4_dev, base);
 }
 
 struct vc4_bo {