drm/i915: Use documented PLL timing limits for G4X platform

The values come from the internal reference spreadsheet on PLL
timing limits for the G4X chipsets.

Part of fixing fd.o bug #17508

Signed-off-by: Ma Ling <ling.ma@intel.com>
[anholt: Cleaned up some whitespace]
Signed-off-by: Eric Anholt <eric@anholt.net>