drm/amdkfd: Implement DMA buf fd export from KFD

drm/amdkfd: Implement DMA buf fd export from KFD

Exports a DMA buf fd of a given KFD buffer handle. This is intended for
being able to import KFD BOs into GEM contexts to leverage the
amdgpu_bo_va API for more flexible virtual address mappings. It will
also be used for the new upstreamable RDMA solution coming to UCX and
RCCL.

The corresponding user mode change (Thunk API and kfdtest) is here:
https://github.com/fxkamd/ROCT-Thunk-Interface/commits/fxkamd/dmabuf

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