drm/vc4: kms: Avoid setting core and disp clocks for hdmi modes

On 2712, the firmware always runs these clock at a speed sufficient
for dual 4kp60.

The requests here prevent the gpu from going into its lowest voltage
mode, so just skip the clock requests.

With this applied the idle voltage on my pi 5 reduces from 0.7424V
to 0.72V.

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