drm/amdgpu/soc15: dynamic initialize ip offset for vega20

Vega20 need a seperate vega20_reg_init.c due to ip base
offset difference.

Reviewed-by: Christian König <christian.koenig@amd.com>
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>