vc4_crtc: Increase setup cost in core clock calculation to handle extreme reduced blanking

See: https://github.com/raspberrypi/linux/issues/4446



The formula that determines the core clock requirement based on pixel clock and blanking
has been determined experimentally to minimise the clock while supporting all modes we've seen.

A new reduced blanking mode (4kp60 at 533MHz rather than the standard 594MHz) has been seen
that doesn't produce a high enough clock and results in "flip_done timed out" error.

Increase the setup cost in the formula to make this work.
The result is a reduced blanking mode increases by up to 7MHz while leaving the standard timing
mode untouched

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