drm/i915: Rename planar linked plane variables

Rename linked_plane to planar_linked_plane and slave to planar_slave,
this will make it easier to keep apart bigjoiner linking and planar plane
linking.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190920114235.22411-8-maarten.lankhorst@linux.intel.com


Reviewed-by: Matt Roper <matthew.d.roper@intel.com>