Fix flicker issue on external display.

Relnote: when primary and virtual display fences are already signaled
while external one is still pending, external display fence should be
overridden by merged fence of primary and virtual.

Bug: 264002998
Test: atest libsurfaceflinger_unittest
Change-Id: Ia1ac1fcc20cfd96d148c549c06777be387df1bde