Turns out G4x needs to have sensible watermarks set, especially for
self-refresh enabled modes.  Add support for it.
Signed-off-by: NJesse Barnes <jbarnes@virtuousgeek.org>
Tested-by: NDirk Hohndel <hohndel@infradead.org>
Signed-off-by: NEric Anholt <eric@anholt.net>