drm/amdgpu: Properly allocate VM invalidate eng v2

v1: Properly allocate TLB invalidation engine to avoid conflict.
v2: Added comments to codes

Signed-off-by: Oak Zeng <Oak.Zeng@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Acked-by: Christian Konig <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org