drm/amdgpu: Add nbio 7.4 support for vega20 (v3)

Some register offset in nbio v7.4 are different with v7.0.
We need a seperate nbio_v7_4.c for vega20.

v2: fix doorbell range for sdma (Alex)
v3: squash in static fix (kbuild test robot)

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>