drm/amdgpu: add support for exporting VRAM using DMA-buf v3

We should be able to do this now after checking all the prerequisites.

v2: fix entrie count in the sgt
v3: manually construct the sg

Signed-off-by: Christian König <christian.koenig@amd.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Sumit Semwal <sumit.semwal@linaro.org>
Link: https://patchwork.freedesktop.org/patch/359295