drm/amdgpu: fix a race in kfd_mem_export_dmabuf()

Using drm_gem_prime_handle_to_fd() to set dmabuf up and insert it into
descriptor table, only to have it looked up by file descriptor and
remove it from descriptor table is not just too convoluted - it's
racy; another thread might have modified the descriptor table while
we'd been going through that song and dance.

Switch kfd_mem_export_dmabuf() to using drm_gem_prime_handle_to_dmabuf()
and leave the descriptor table alone...

Reviewed-by: Felix Kuehling <felix.kuehling@amd.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>