[PATCH] Fixes for interaction of imported dma-bufs and scanout

For nouveau, radeon, and amdgpu attempting to scanout of a bo will cause the driver to migrate it to VRAM.

When the bo in question is an imported dma-buf, this results in a buffer that appears to be shared but is actually device-private.

Consensus on #dri-devel seemed to be that it would be appropriate for addfb to fail when attempted on an imported dma-buf.

It *might* also be appropriate to fail addfb on an exported dma-buf (as that is likewise pinned to GTT), but #dri-devel thought it possible the buffer could be unshared between addfb and actually trying to scanout. An imported dma-buf doesn't have the same problem.