HWC2: Fix virtual displays

Close the output buffer release fence per the contract in
hwcomposer2.h.

Also changes the HWC2 device to hold a weak pointer to the display,
which allows it to properly remove it when it gets torn down.

Test: Cherry-pick from internal branch
Bug: 29945684
Change-Id: Ib1b5d81e42a63d0e65b189f7c6c62e661e0ab6ae
(cherry picked from commit 5450fe52)