drm/radeon: take the mode_config mutex when dealing with hpds (v2)

commit 39fa10f7e21574a70cecf1fed0f9b36535aa68a0 upstream.

Since we are messing with state in the worker.

v2: drop the changes in the mst worker

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>