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.

Reviewed-by: Dillon Varone <dillon.varone@amd.com>
Signed-off-by: Brendan Tam <Brendan.Tam@amd.com>
Signed-off-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org