drm/i915: Use the stored cursor and plane latencies properly

Rather than pass around the plane latencies, just grab them from
dev_priv nearer to where they're needed. Do the same for cursor
latencies.

v2: Add some comments about latency units

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>