sr: Fix vsync logic

 * Use CLOCK_MONOTONIC to insulate from system clock changes.
 * Normalize all timespec calculations to avoid overflow/underflow.
 * Don't signal vsync if poll() fails.

Change-Id: If284ebf581309953c51b2f8d33d7d5800c636be5
(cherry picked from commit b6fbab00cfea400915399b97a9644870edfb64b7)