drm/amdgpu: handle SRIOV VCN revision parsing

For SR-IOV, the IP discovery revision number encodes
additional information.  Handle that case here.

v2: drop additional IP versions

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