drm/amdgpu: Properly allocate VM invalidate eng v2

commit c5066129af4436ab0da8eefe4289774a5409706d upstream.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>