drm/radeon: move ring syncing after bo validation

The function radeon_bo_list_validate can cause a
bo to move, resulting in a different sync_obj
and a dependency to wait for this move to finish.

Signed-off-by: Christian König <deathsimple@vodafone.de>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Jerome Glisse <jglisse@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>