Fixes crash on MST Hub disconnect.

Bug: https://gitlab.freedesktop.org/drm/amd/-/issues/1849
Fixes: ee2698cf ("drm/amd/display: Changed pipe split policy to allow for multi-display pipe split")
Signed-off-by: Benjamin Marty <info@benjaminmarty.ch>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>