drm/i915: Add SNB/IVB support to intel_read_wm_latency

SNB and IVB have slightly a different way to read out the
watermark latency values.

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>