drm/vc4: plane: Switch to drmm_universal_plane_alloc()

Let's switch to drmm_universal_plane_alloc() for our plane allocation and
initialisation to make the driver a bit simpler.

Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Dave Stevenson <dave.stevenson@raspberrypi.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>