drm/i915: Make G4X-style PLL search more permissive
Fixes an Ironlake laptop with a 68.940MHz 1280x800 panel and 120MHz SSC
reference clock.
More generally, the 0.488% tolerance used before is just too tight to
reliably find a PLL setting. I extracted the search algorithm and
modified it to find the dot clocks with maximum error over the valid
range for the given output type: