drm/vc4: crtc: Move crtc state to common header

We'll need to access the crtc_state from outside of vc4_crtc.c, so let's
move it to vc4_drv.h

Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/1e6e563f9c75961e2885c9d648a3130d3b46b6d1.1590594512.git-series.maxime@cerno.tech