drm/amd/display: adjust flow for deallocation mst payload

[Why]
MST relevant variables are maintained at drm side. As the result, we
still have to call drm_dp_remove_payload_part2() to update the relevant
values regardless the link is under mst mode or not. We used to have a
workaround patch to tackle this: commit 3d8fcc67 ("drm/amd/display:
Extract temp drm mst deallocation wa into its own function")

Now it's time to remove the workaround and adjust the flow.

[How]
During deallocate_mst_payload(), source actually doesn't send out
ALLOCATE_PAYLOAD at the end as like the flow in allocate_mst_payload().
Call function dm_helpers_dp_mst_send_payload_allocation() at the end of
deallocate_mst_payload() is a bit confusing.

Separate dm_helpers_dp_mst_send_payload_allocation() into 2 functions.
Have a new function dm_helpers_dp_mst_update_mst_mgr_for_deallocation()
to replace dm_helpers_dp_mst_send_payload_allocation() for payload
deallocation.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Wenjing Liu <wenjing.liu@amd.com>
Acked-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: Wayne Lin <wayne.lin@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>