drm/amdgpu: limit the VM address space with older VCE FW versions

Older VCE FW versions are buggy and can't work with 48bit address
spaces.

RFC: Should we limit the address space or just reject loading the older
VCE firmware?

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>