drm/vc4: crtc: Make encoder lookup helper public

We'll need a function that looks up an encoder by its vc4_encoder_type.
Such a function is already present in the CRTC code, so let's make it
public so that we can reuse it in the unit tests.

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-v3-15-4615a663a84a@cerno.tech
Signed-off-by: Maxime Ripard <maxime@cerno.tech>