drm/vc4: fkms to query the VPU for HDMI clock limits

The VPU has configured clocks for 4k (or not) via config.txt,
and will limit the choice of video modes based on that.
Make fkms query it for these limits too to avoid selecting modes
that can not be handled by the current clock setup.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.org>