drm/vc4: hdmi: Use the connector state pixel rate for the PHY

The PHY initialisation parameters are not based on the pixel clock but
the TMDS clock rate which can be the pixel clock in the standard case,
but could be adjusted based on some parameters like the bits per color.

Since the TMDS clock rate is stored in our custom connector state
already, let's reuse it from there instead of computing it again.

Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Dave Stevenson <dave.stevenson@raspberrypi.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>