drm/amd/display: Add dmcu f/w loading for NV12

[Why]
We need DMCU for features like PSR and ABM.

[How]
Add path to dmcu firmware binary and load it for Navi12.

Signed-off-by: Roman Li <roman.li@amd.com>
Reviewed-by: Hersen Wu <hersenxs.wu@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>