drm/amdgpu: Fail DMABUF map of XGMI-accessible memory

If peer memory is XGMI-accessible, we should never access it through PCIe
P2P DMA mappings. PCIe P2P is slower, has different coherence behaviour,
limited or no support for atomics, or may not work at all. Fail with a
warning if DMABUF mappings of such memory are attempted.

Signed-off-by: Felix Kuehling <felix.kuehling@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>