drm/vc4: hdmi: Move structure to header

drm/vc4: hdmi: Move structure to header

We will need to share the vc4_hdmi and related structures with multiple
files, so let's create a header for it.

Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Tested-by: Chanwoo Choi <cw00.choi@samsung.com>
Tested-by: Hoegeun Kwon <hoegeun.kwon@samsung.com>
Tested-by: Stefan Wahren <stefan.wahren@i2se.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/db3ff2babc914e38e86f794c4e49128697aed887.1599120059.git-series.maxime@cerno.tech