drm/amd/display: Unlock on error path in dm_handle_mst_sideband_msg_ready_event()

This error path needs to unlock the "aconnector->handle_mst_msg_ready"
mutex before returning.

Fixes: 4f6d9e38 ("drm/amd/display: Add polling method to handle MST reply packet")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>