drm/amdgpu: Move to common indirect reg access helper

Replace soc15, nv, soc21 specific callbacks with common
one. so we don't need to duplicate code when introduce
new asics.

Signed-off-by: Hawking Zhang <Hawking.Zhang@amd.com>
Reviewed-by: Likun Gao <Likun.Gao@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>