drm/i915: Allow preservation of watermarks, v2.

As Paulo has noted we can help bisectability by separating computing
watermarks on a noop in 2 separate commits.

This patch no longer clears the crtc watermark state, but recalculates
it completely. Regardless whether a level is used the full values for
each level are calculated. If a level is invalid wm[level].enable is
unset.

Changes since v1:
- Only call ilk_validate_wm_level when level <= usable_level. (Ville)

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/56D6D09E.5040007@linux.intel.com


Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>