drm/amd/display: Refactor fixed VS w/a for PHY tests

drm/amd/display: Refactor fixed VS w/a for PHY tests

[Why/How]
Refactor original w/a to unify naming and
simplify logic. This also re-enables the code
that was previously skipped due to the
disabling of the previous workaround logic.

Reviewed-by: Wenjing Liu <Wenjing.Liu@amd.com>
Reviewed-by: Nevenko Stupar <Nevenko.Stupar@amd.com>
Acked-by: Alan Liu <HaoPing.Liu@amd.com>
Signed-off-by: George Shen <George.Shen@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>