Migration to VRAM will break the sharing, resulting in rendering on the exporting GPU never becoming
visible on the importing GPU.

v2: Don't pin BOs to GTT. Instead, refuse to migrate them out of GTT.
Reviewed-by: NChristian König <christian.koenig@amd.com>
Signed-off-by: NChristopher James Halse Rogers <christopher.halse.rogers@canonical.com>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>