drm/amd/display: prevent hang on link training fail

In the Linux kernel, the following vulnerability has been resolved: drm/amd/display: prevent hang on link training fail [Why] When link training fails, the phy clock will be disabled. However, in enable_streams, it is assumed that link training succeeded and the mux selects the phy clock, causing a hang when a register write is made. [How] When enable_stream is hit, check if link training failed. If it did, fall back to the ref clock to avoid a hang and keep the system in a recoverable state.