drm/i915: Refactor wm_lp to level calculation

On HSW the LP1,LP2,LP3 levels are either 1,2,3 or 1,3,4. We make the
conversion from LPn to to the level at one point current. Later we're
going to do it in a few places, so move it to a separate function.

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>