drm/amdgpu: don't reemit if there is nothing to reemit

drm/amdgpu: don't reemit if there is nothing to reemit

Return early in amdgpu_ring_set_fence_errors_and_reemit()
if ring_backup_entries_to_copy is 0.  That means that either
the ring is idle and there is nothing to reemit, or there
some reason why we should reemit, so return early and
signal the fences (if applicable).

Reviewed-by: Jesse Zhang <jesse.zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>