Fix BlockUntilVSync() to actually block

In my refactoring in ag/2008740 I introduced a regression where blocking
until the next vsync would busy loop instead of actually blocking on an
event on the vsync fd. The problem was I changed the poll() call to
listen for POLLIN in addition to POLLPRI. While waiting for vsync events
we only want to listen for POLLPRI.

Bug: 36495351

Test: Confirmed with debug logs the busy loop bug and that we correctly
block with this patch applied.

Change-Id: Iec4951e014575a5d0ed3cfe3fc20ea91c67edf1b