pipe A turned off would then also force all outputs
on pipe B to get turned off as the disable code would mistakenly
decide that all of these outputs were on pipe A and turn them off.

This is an extension of the CPT DP disable fix (why didn't I fix this then?)

Signed-off-by: Keith Packard <keithp@keithp.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
/w