Rework the management of vblank interrupts a bit.

When a vt switch occurs the irq handler is uninstalled.  Interrupts
and the state tracking of what was enabled/disabled wasn't working
properly.  This should resolve the reports of "slow windows" after a
vt switch, among other things.  The radeon 2d driver seems to work a
bit more correctly than the Intel driver.  With the Intel driver,
vblank interrupts will be enabled at system startup and will only
be disabled after an additional modeset (vt switch, dpms, randr event).

With this patch, I am able to run glxgears synced to vblank and
vt switch while it is running without ill effects.

MFC after:	3 days

0 files changed, 0 insertions, 0 deletions