drm/i915: update watermarks before enabling PLLs

When coming back from DPMS or turning on a display, make sure we have
the watermarks set up before turning on the display plane, otherwise we
may get underruns.

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