drm/i915: apply G45 vblank count code to all G4x chips and fix max_frame_count

All G4x and newer chips use the new style frame count register, with a
full 32 bit frame count.  Update the code to reflect this.

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Eric Anholt <eric@anholt.net>