drm/vc4: crtc: Introduce a lower-level crtc init helper

The current vc4_crtc_init() helper assumes that we will be using
hardware planes and calls vc4_plane_init().

While it's a reasonable assumption, we'll want to mock the plane and
thus provide our own. Let's create a helper that will take the plane as
an argument.

Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Reviewed-by: Maíra Canal <mcanal@igalia.com>
Link: https://lore.kernel.org/r/20221123-rpi-kunit-tests-v3-14-4615a663a84a@cerno.tech
Signed-off-by: Maxime Ripard <maxime@cerno.tech>