drm/vc4: hdmi: Implement a register layout abstraction

The HDMI controllers found in the BCM2711 have most of the registers
reorganized in multiple registers areas and at different offsets than
previously found.

The logic however remains pretty much the same, so it doesn't really make
sense to create a whole new driver and we should share the code as much as
possible.

Let's implement some indirection to wrap around a register and depending on
the variant will lookup the associated register on that particular variant.

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