That allows the VM code to use GART BOs from other driver instances.

v2: don't use copy optimization for foreign GARTs, that won't work.
v3: some more comment cleanups
v4: agd: rebase on upstream
Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>