Ignore display state changes for disconnected displays

When a display is disconnected, removing it from SurfaceFlinger's
display list is non-atomic with removing it from the Display Manager
and any in-flight transactions. So SurfaceFlinger might get a display
state change transaction for a display it has already forgotten about.
Just ignore these.

Bug: 7288082
Change-Id: Ic27e55377f3db40fb34e3b1cd67e43297df117a2