drm/amdgpu: add FENCE_TO_HANDLE ioctl that returns syncobj or sync_file

for being able to convert an amdgpu fence into one of the handles.
Mesa will use this.

Reviewed-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Marek Olšák <marek.olsak@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>