drm/mst: Fix NULL pointer dereference at drm_dp_add_payload_part2

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().

Fixes: 5aa1dfcdf0a4 ("drm/mst: Refactor the flow for payload allocation/removement")
Reported-by: Leon Weiß <leon.weiss@ruhr-uni-bochum.de>
Link: https://lore.kernel.org/r/38c253ea42072cc825dc969ac4e6b9b600371cc8.camel@ruhr-uni-bochum.de/
Cc: lyude@redhat.com
Cc: imre.deak@intel.com
Cc: stable@vger.kernel.org
Cc: regressions@lists.linux.dev
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Acked-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Wayne Lin <Wayne.Lin@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240307062957.2323620-1-Wayne.Lin@amd.com