drm/i915/gen9: Calculate plane WM's from state

In a future patch we'll want to calculate plane watermarks for in-flight
atomic state rather than the already-committed state.

Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1463061971-19638-12-git-send-email-matthew.d.roper@intel.com