drm/i915: Update watermarks for Ironlake after dpms changes

Previously, we only remembered to update the watermarks for i9xx, and
incorrectly assumed that the crtc->enabled flag was valid at that point
in the dpms cycle.

Note that on my x201s this makes a SR bug on pipe 1 much easier to hit.
(Since before this patch when disabling pipe 0, we either didn't update
the watermarks at all, or when we did we still thought we had two pipes
enabled and so disabled SR.)

References:

  Bug 28969 - [Arrandale] Screen flickers, suspect Self-Refresh
  https://bugs.freedesktop.org/show_bug.cgi?id=28969



Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Eric Anholt <eric@anholt.net>