Trying to fix drm/prime: remove drm_prime_lookup_buf_by_handle

[Christian König]

The problem with this patch is that we now don't check if the handle
already existed any more, but just blindly try to add it again.

This obviously breaks re-exporting DMA-bufs but also surfaces an issue
that we don't seem to take into account that multiple handles can point
to the same GEM object.

Try to fix this by just ignoring adding the same DMA-buf to the rb tree
multiple times. Not sure if that is the best approach, but it should fix
the issue at hand.

Regards,
Christian.