drm/vc4: Make vc4_crtc_get_encoder public

We'll need that function in vc4_kms to compute the core clock rate
requirements.

Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Reviewed-by: Dave Stevenson <dave.stevenson@raspberrypi.com>
Link: https://lore.kernel.org/r/20211025152903.1088803-4-maxime@cerno.tech