drm/vc4: hdmi: Move accessors to vc4_hdmi

The current driver only supports a single HDMI controller, and part of
the issue is that the main vc4_dev structure holds a pointer to its
(only) HDMI controller, and the HDMI registers accessors will use it to
retrieve the mapped addresses.

Let's modify those accessors to use directly the vc4_hdmi structure so
that we can eventually get rid of that single global pointer.

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/886b955586264ce078d7d35e9b8ef9ae51675c27.1599120059.git-series.maxime@cerno.tech