drm/armada: Remove prepare/cleanup_fb hooks

All they do is refcount the fb, which the atomic helpers already do.

This is was necessary with the legacy helpers and I guess just carry
over in the conversion. drm_plane_state always has a full reference
for its ->fb pointer during its entire lifetime,
see __drm_atomic_helper_plane_destroy_state()

Acked-by: Maxime Ripard <maxime@cerno.tech>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Russell King <linux@armlinux.org.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20210622165511.3169559-10-daniel.vetter@ffwll.ch