vc4_hdmi_phy: Fix offset calculation

The original firmware code worked with float and did
   offset = ((vco_freq / fref * 2) * (1 << 22));
   offset >>= 2;

In this code it's all integer so doing the integer divide before the shift loses lots of precision

This fixes the issue of 1080p59.94 mode having 59.64 fps

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