drm/amdgpu/dm/mst: Fix uninitialized var in pre_compute_mst_dsc_configs_for_state()

Coverity noticed this one, so let's fix it.

Fixes: ba891436 ("drm/amdgpu/mst: Stop ignoring error codes and deadlocking")
Signed-off-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Cc: stable@vger.kernel.org # v5.6+