drm/amd/display: Update FIXED_VS Link Rate Toggle Workaround Usage

[WHY]
Previously the 128b/132b LTTPR support DPCD field was used to decide if
FIXED_VS training sequence required a rate toggle before initiating LT.

When running DP2.1 4.9.x.x compliance tests, emulated LTTPRs can report
no-128b/132b support which is then forwarded by the FIXED_VS retimer.
As a result this test exposes the rate toggle again, erroneously causing
failures as certain compliance sinks don't expect this behaviour.

[HOW]
Add new DPCD register defines/reads to read LTTPR IEEE OUI and device ID.

Decide whether to perform the rate toggle based on the LTTPR's IEEE OUI
which guarantees that we only perform the toggle on affected retimers.

Reviewed-by: Wenjing Liu <wenjing.liu@amd.com>
Signed-off-by: Michael Strauss <michael.strauss@amd.com>
Signed-off-by: Zaeem Mohamed <zaeem.mohamed@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>