drm/amd/display: Ensure link output is disabled in backend reset for PLL_ON

drm/amd/display: Ensure link output is disabled in backend reset for PLL_ON

[Why]
We're missing the code to actually disable the link output when we have
to leave the SYMCLK_ON but the TX remains OFF.

[How]
Port the code from DCN401 that detects SYMCLK_ON_TX_OFF and disable
the link output when the backend is reset.

Reviewed-by: Ovidiu (Ovi) Bunea <ovidiu.bunea@amd.com>
Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Signed-off-by: Matthew Stewart <matthew.stewart2@amd.com>
Tested-by: Dan Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>