drm/i915/dp: Master/Slave enable/disable sequence for bigjoiner

drm/i915/dp: Master/Slave enable/disable sequence for bigjoiner

Enabling is done in a special sequence and so should plane updates
be. Ideally the end user never notices the second pipe is used.

This way ideally everything will be tear free, and updates are
really atomic as userspace expects it.

This uses generic modeset_enables() calls like trans port sync
but still has special handling for disable since for slave we
should not disable things like encoder, plls that are not enabled
for  slave.

Signed-off-by: Manasi Navare <manasi.d.navare@intel.com>
[vsyrjala: Appease checkpatch]
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Animesh Manna <animesh.manna@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201117194718.11462-6-manasi.d.navare@intel.com