drm/amdkfd: Access gpuvm_export_dmabuf() API to get Dmabuf

Directly invoking the function amdgpu_gem_prime_export() from within
KFD is not correct. By utilizing the KFD API to obtain Dmabuf, the
implementation can prevent the creation of multiple instances of
struct dma_buf.

Signed-off-by: Ramesh Errabolu <Ramesh.Errabolu@amd.com>
Reviewed-by: David Francis <David.Francis@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>