drm/i915/display: Check the old state to find port sync slave

drm/i915/display: Check the old state to find port sync slave

If the CRTC is going from enabled to disabled and it is a port sync
slave, it needs to check to the old state to be disabled before the
port sync master.

Cc: Manasi Navare <manasi.d.navare@intel.com>
Cc: Matt Roper <matthew.d.roper@intel.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191202222513.337777-1-jose.souza@intel.com