drm/i915: Improve watermark dirtyness checks

Currently hsw_write_vm_values() may write to certain watermark
registers needlessly. For instance if only, say, LP3 changes,
the current code will again disable all LP1+ watermarks even
though only LP3 needs to be reconfigured.

Add an easy to read function that will compute the dirtyness of the
watermarks, and use that information to further optimize the watermark
programming.

v2: Disable LP1+ watermarks around changing LP0 watermarks for Paulo

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>