graphics: really reset HWC2 on SF crashes

Perform a final presentDisplay to clear the screen and get HWC2 out
of any special state (e.g., some implementations put themselves in
special states between validateDisplay and presentDisplay).  Our
only portable choice really is to abort(), but that is not an option
because we have other clients such as VTS or VR.

Bug: 35872161
Test: manual
Change-Id: I9028705607d1b86d418a379c6e90e833d638b3f5