drm/i915: Store the watermark latency values in dev_priv

Rather than having to read the latency values out every time, just
store them in dev_priv.

On ILK and IVB there is a difference between some of the latency
values for different planes, so store the latency values for each
plane type separately, and apply the necesary fixups during init.

v2: Fix some checkpatch complaints

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>