drm/amd/display: Add DCE_VERSION_12_1 enum for Vega 20

[Why]
We'll need a way to differentiate Vega 20 in DC

[How]
Add a DCE_VERSION_12_1 enum, which will be returned as the DC version if
the ASIC used is a Vega 20.

Signed-off-by: Leo Li <sunpeng.li@amd.com>
Reviewed-by: David Francis <David.Francis@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>