fix a corruption in blank/unblank

we were holding a reference (ie: pointer) to a sp<DisplayDevice>
while processing the message. Meanwhile the object itself could
go away and we would end up accessing a dead object.

the root cause of the problem is that we are accessing mDisplays[]
in a few places outside of the main thread.

Bug: 7352770
Change-Id: I89e35dd85fb30e9a6383eca9a0bbc7028363876c