drm/vc4: hdmi: Fix clock value used for validating hdmi modes

We are using mode->crt_clock here which is filled by drm_mode_set_crtcinfo()
which is called right after .mode_valid.

Use mode->clock which is valid here.

Fixes: 624d93a4 ("drm/vc4: hdmi: Move clock calculation into its own function")

Signed-off-by: Dom Cobley <popcornmix@gmail.com>