drm/vc4: hdmi: Check the HSM rate at runtime_resume

If our HSM clock has not been properly initialized, any register access
will silently lock up the system.

Let's check that this can't happen by adding a check for the rate before
any register access, and error out otherwise.

Link: https://lore.kernel.org/dri-devel/20220922145448.w3xfywkn5ecak2et@pengutronix.de/
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Tested-by: Stefan Wahren <stefan.wahren@i2se.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20220929-rpi-pi3-unplugged-fixes-v1-2-cd22e962296c@cerno.tech