drm/amd/display: Clean up fixed VS PHY test w/a function

[Why]
Unify naming for fixed VS workarounds.

[How]
Rename function to match naming convention.
Refactor code to remove unused function.

Reviewed-by: Wenjing Liu <Wenjing.Liu@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>