drm/amdgpu: move vmhub out of amdgpu_ring_funcs (v4)

It looks better to place this field in ring
structure. Also drop the repeated ring funcs definitions
if there's no difference except for vmhub field.

v2: rename the field to vm_hub like others (Le)
v3: apply the changes to new ip blocks (Hawking)
v4: fix vcn sw ring (Alex)

Signed-off-by: Le Ma <le.ma@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>