drm/amd/display: Don't perform rate toggle on DP2-capable FIXED_VS retimers

[WHY]
Only required if FIXED_VS retimer does not support DP2-capable.

[HOW]
Gate link rate toggle with DP 128b/132b LTTPR channel coding cap check.

Reviewed-by: Charlene Liu <charlene.liu@amd.com>
Acked-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: Michael Strauss <michael.strauss@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>