drm/amd/display: Improve FAM control for DCN401

[why & how]
When the commit 5324e2b2 ("drm/amd/display: Add driver support for
future FAMS versions") was introduced, it missed some of the FAM2 code.
This commit introduces the code that control the FAM enable and disable.

Fixes: 5324e2b2 ("drm/amd/display: Add driver support for future FAMS versions")
Acked-by: Wayne Lin <wayne.lin@amd.com>
Signed-off-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Tom Chung <chiahsuan.chung@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>