drm/amdgpu: convert nbio to use callbacks (v2)

Cleans up and consolidates all of the per-asic logic.

v2: squash in "drm/amdgpu: fix NULL err for sriov detect" (Chunming)

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