drm/vc4: hdmi: Warn if writing to an unknown HDMI register

The VC4 HDMI driver has a bunch of accessors to read from a register.
The read accessor was warning when accessing an unknown register, but
the write one was just returning silently.

Let's make sure we warn also when writing to an unknown register.

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