drm/amdgpu: Add nbio support for vega20 (v2)

Some register offset in nbio v7.4 are different with v7.0.

v2: Use nbio7.0 for now.

TODO: add a new nbio 7.4 module (Alex)

Signed-off-by: Feifei Xu <Feifei.Xu@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>