drm/vc4: Set a default HSM rate

When the firmware doesn't setup the HSM rate (such as when booting
without an HDMI cable plugged in), its rate is 0 and thus any register
access results in a CPU stall, even though HSM is enabled.

Let's enforce a minimum rate at boot to avoid this issue.

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