Add an DMA-buf export implementation independent of the DRM helpers.

This not only avoids the caching of DMA-buf mappings, but also
allows us to use the new dynamic locking approach.

This is also a prerequisite of unpinned DMA-buf handling.

v2: fix unintended recursion, remove debugging leftovers
v3: split out from unpinned DMA-buf work
v4: rebase on top of new no_sgt_cache flag
v5: fix some warnings by including amdgpu_dma_buf.h
v6: fix locking for non amdgpu exports
v7: rebased on new DMA-buf locking patch
v8: drop extra include
Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Link: https://patchwork.freedesktop.org/patch/337949/