drm/radeon: Refuse to migrate a prime BO to VRAM. (v2)

BOs shared via dma-buf, either imported or exported, cannot sensibly be migrated to VRAM
without breaking the dma-buf sharing. Refuse userspace requests to migrate to VRAM,
ensure such BOs are not migrated during command submission, and refuse to pin them
to VRAM.

v2: Don't pin BOs in GTT. Instead, refuse to migrate BOs to VRAM.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Christopher James Halse Rogers <christopher.halse.rogers@canonical.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>