drm/amdgpu/mst: Stop ignoring error codes and deadlocking

It appears that amdgpu makes the mistake of completely ignoring the return
values from the DP MST helpers, and instead just returns a simple
true/false. This is bad because error codes are important. In this case, it
seems to have come back to bite us because as a result of simply returning
false from compute_mst_dsc_configs_for_state(), amdgpu had no way of
telling when a deadlock happened from these helpers. This results in the
latest kernel splat with TB hotplugging on amdgpu.

Signed-off-by: Lyude Paul <lyude@redhat.com>
Fixes: 8c20a1ed ("drm/amd/display: MST DSC compute fair share")
Cc: Harry Wentland <harry.wentland@amd.com>
Cc: <stable@vger.kernel.org> # v5.6+