drm/amd/display: Synchronous DisplayPort Link Training

[WHY]
We require a method to perform synchronous link training.

[HOW]
Sync LT is broken into 3 basic steps.
"Begin" starts the state machine, and resets "preferred" link settings.
"Attempt" will attempt to train the link with a given set of training
parameters.
"End" stops the state machine, and will optionally disable the link phy.
Between "Begin" and "End" DPCD:600h must not be set to "2"
(D3:Powered Down).
Between "Begin" and "End", there may be multiple "Attempts" with different
training parameters.

Signed-off-by: David Galiffi <david.galiffi@amd.com>
Reviewed-by: Jun Lei <Jun.Lei@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>