drm/i915: Refactor wm calculations

All platforms until SKL compute their watermarks essentially
using the same method1/small buffer and method2/large buffer
formulas. Most just open code it in slightly different ways.
Let's pull it all into common helpers. This makes it a little
easier to spot the actual differences.

While at it try to add some docs explainign what the formulas
are trying to do.
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170421181432.15216-11-ville.syrjala@linux.intel.comReviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>