drm/amd/display: Fix possible infinite loop in DP LT fallback

[Why]
It's possible for some fallback scenarios to result in infinite looping
during link training.

[How]
This change modifies DP LT fallback behavior to more closely match the
DP standard. Keep track of the link rate during the EQ_FAIL fallback,
and use it as the maximum link rate for the CR sequence.

Reviewed-by: Wenjing Liu <Wenjing.Liu@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Ilya <Ilya.Bakoulin@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>