drm/vc4: Make FKMS max refresh rate a module parameter

Some people want to use the high refresh rate modes for 1080p100
and 1080p120, but they're currently filtered out as generally
they don't add anything.

Make the filter threshold a module parameter so that it can be
adjusted.

https://github.com/raspberrypi/linux/issues/3677



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