drm/i915: HW state readout for Bigjoiner case

Skip iterating over bigjoiner slaves, only the master has the state we
care about.

Add the width of the bigjoiner slave to the reconstructed fb.

Hide the bigjoiner slave to userspace, and double the mode on bigjoiner
master.

And last, disable bigjoiner slave from primary if reconstruction fails.

v3:
* Fix the ddi_get_config slave error (Ankit Nautiyal)
v2:
* Unsupported bigjoiner config for initial fb (Ville)

Signed-off-by: Manasi Navare <manasi.d.navare@intel.com>
[vsyrjala:
* Don't do any hw->uapi state copy for bigjoiner slave
* We still have hw.mode so no need to pass it in
* 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/20201117194718.11462-7-manasi.d.navare@intel.com