drm/vc4: crtc: Add encoder to vc4_crtc_config_pv prototype

vc4_crtc_config_pv() retrieves the encoder again, even though its only
caller, vc4_crtc_atomic_enable(), already did.

Pass the encoder pointer as an argument instead of going through all the
connectors to retrieve it again.

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