Avoid incorrect vblank wakeups

The current code uses the hw vblank counter exclusively, which can lead
to wakeups during the active period rather than during the vblank period
if the hw counter counts displayed frames rather than vblank periods.

This change coverts the code over to using the counter while interrupts
are enabled, fixing that issue.  It also includes a couple of related
changes:  one to not enable the new enable/disable behavior until the
modeset ioctl is called (to preserve old client behavior) and another to
account for lost events due to mode setting with the new counter scheme.

BSD will require similar changes to its drm_irq.c code, but they should
be straightforward.