drm/vc4: plane: Register a different drm_plane_helper_funcs on BCM2711

On the BCM2711, our current definition of drm_plane_helper_funcs uses
the custom vc4_prepare_fb() and vc4_cleanup_fb().

Those functions rely on the buffer allocation path that was relying on
the GPU, and is no longer relevant.

Let's create another drm_plane_helper_funcs structure that we will
register on the BCM2711.

Signed-off-by: Maxime Ripard <maxime@cerno.tech>