drm/i915: add FIFO watermark support for G4x

Turns out G4x needs to have sensible watermarks set, especially for
self-refresh enabled modes.  Add support for it.

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Tested-by: Dirk Hohndel <hohndel@infradead.org>
Signed-off-by: Eric Anholt <eric@anholt.net>