drm/i915: Use a different PLL timing search function on G4X.

This improves the PLL timings according to the suggestion of the hardware
engineers.  This results in some outputs being able to sync that weren't
able to before.

This is part of fixing fd.o bug #17508.

Signed-off-by: Ma Ling <ling.ma@intel.com>
[anholt: cleaned up a couple of redundant comments]
Signed-off-by: Eric Anholt <eric@anholt.net>