drm/vc4: hdmi: Add a spinlock to protect register access

The vc4 HDMI driver has multiple path shared between the CEC, ALSA and
KMS frameworks, plus two interrupt handlers (CEC and hotplug) that will
read and modify a number of registers.

Even though not bug has been reported so far, it's definitely unsafe, so
let's just add a spinlock to protect the register access of the HDMI
controller.

Fixes: c8b75bca ("drm/vc4: Add KMS support for Raspberry Pi.")
Signed-off-by: Maxime Ripard <maxime@cerno.tech>