drm/amd/display: enable abm on dcn2

[Why]
ABM is currently not enabled on DCN2.

[How]
Update the register name list for DCN2 and un-comment the code that
creates the abm object.

Signed-off-by: Josip Pavic <Josip.Pavic@amd.com>
Reviewed-by: Anthony Koo <Anthony.Koo@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Acked-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>