drm/vc4: hdmi: Add more checks for 4k resolutions

At least the 4096x2160@60Hz mode requires some overclocking that isn't
available by default, even if hdmi_enable_4kp60 is enabled.

Let's add some logic to detect whether we can satisfy the core clock
requirements for that mode, and prevent it from being used otherwise.

Signed-off-by: Dom Cobley <popcornmix@gmail.com>
Reviewed-by: Dave Stevenson <dave.stevenson@raspberrypi.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>