sdm: check for more than one vsync callback handling

Long preemption of SDM Event thread can cause handling
of more than one vsync handling from libdrm. Check for
this condition and re-enable vsync if needed.

Bug: 157708435
Test: 60-90hz transition stress test
Test: AoD in and out stress test
Change-Id: Ie045fd80d27e5e1b718b50d4a7b63c85ff17db87
2 files changed