drm/vc4: crtc: Make state functions public

We'll need the CRTC state related functions to be exported so that we can
reuse them for the TXP.

Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Reviewed-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/658f40aa01d7a45cbf6feebfc3dc6549f100d110.1591882579.git-series.maxime@cerno.tech