drm/i915: Add debugfs dumping for bigjoiner, v3.

Dump debugfs and planar links as well, this will make it easier to debug
when things go wrong.

v4:
* Rebase
Changes since v1:
- Report planar slaves as such, now that we have the plane_state switch.
Changes since v2:
- Rebase on top of the new plane format dumping

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Manasi Navare <manasi.d.navare@intel.com>
Reviewed-by: Animesh Manna <animesh.manna@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201117194718.11462-12-manasi.d.navare@intel.com