drm/mst: Fix NULL pointer dereference at drm_dp_add_payload_part2

In the Linux kernel, the following vulnerability has been resolved:

drm/mst: Fix NULL pointer dereference at drm_dp_add_payload_part2

[Why]

Commit:

- commit 5aa1dfcdf0a4 ("drm/mst: Refactor the flow for payload allocation/removement")

accidently overwrite the commit

- commit 54d217406afe ("drm: use mgr->dev in drm_dbg_kms in drm_dp_add_payload_part2")

which cause regression.

[How]

Recover the original NULL fix and remove the unnecessary input parameter 'state' for

drm_dp_add_payload_part2().

(cherry picked from commit 4545614c1d8da603e57b60dd66224d81b6ffc305)