drm/vc4: hdmi: Replace hardcoded value by define

The 120MHz value hardcoded in the call to max_t to compute the HSM rate
is defined in the driver as HSM_MIN_CLOCK_FREQ, let's switch to it so
that it's more readable.

Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://lore.kernel.org/r/20230126-rpi-display-fw-clk-cleanup-v1-1-d646ff6fb842@cerno.tech


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