drm/i915: Add ILK/SNB/IVB WM latency field support

Add a new function  ilk_wm_lp_latency() which will tell us what to write
into the WM_LPx register latency field. HSW is different from erlier
gens in this regard.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>