drm: Update MST First Link Slot Information Based on Encoding Format

8b/10b encoding format requires to reserve the first slot for
recording metadata. Real data transmission starts from the second slot,
with a total of available 63 slots available.

In 128b/132b encoding format, metadata is transmitted separately
in LLCP packet before MTP. Real data transmission starts from
the first slot, with a total of 64 slots available.

v2:
* Move total/start slots to mst_state, and copy it to mst_mgr in
atomic_check

v3:
* Only keep the slot info on the mst_state
* add a start_slot parameter to the payload function, to facilitate non
  atomic drivers (this is a temporary workaround and should be removed when
  we are moving out the non atomic driver helpers)

v4:
*fixed typo and formatting

v5: (no functional changes)
* Fixed formatting in drm_dp_mst_update_slots()
* Reference mst_state instead of mst_state->mgr for debugging info

Signed-off-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Fangzhi Zuo <Jerry.Zuo@amd.com>
[v5 nitpicks]
Reviewed-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Lyude Paul <lyude@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211025223825.301703-3-lyude@redhat.com