modesetting: Skip no-longer-present connectors when resetting BAD links

Outputs may have NULL mode_output (connector) pointers if the
connector disappears while the server is running. Skip these when
resetting outputs with BAD link status.

Signed-off-by: Keith Packard <keithp@keithp.com>
Reviewed-by: Adam Jackson <ajax@redhat.com>
(cherry picked from commit 37f4e765)