drm/i915: Use the watermark latency values from dev_priv for ILK/SNB/IVB too

Adjust the current ILK/SNB/IVB watermark codepaths to use the
pre-populated latency values from dev_priv instead of reading
them out from the registers every time.

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>