drm/amd/display: clean up DML for DCN2x

[why]
Previous "less risky" implemenation of 3 tiered fallback is no longer necessary since
DMLv2 has gone through proper validation.  v2 can now be used as the default and 1
level of fallback can be removed

[how]
remove previous workaround implemenation

Signed-off-by: Jun Lei <Jun.Lei@amd.com>
Reviewed-by: Nicholas Kazlauskas <Nicholas.Kazlauskas@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>