sna: Avoid recursive out-of-order operations in the middle of TearFree

sna: Avoid recursive out-of-order operations in the middle of TearFree

During TearFree, if we have to wait for the shadow flip to complete, we
run the event handler for the device. This can then cause us to evaluate
pending completed vblank events, which may in turn then attempt to use
the shadow bo and recurse into the TearFree handler. Try to prevent this
and requeue the vblank event.

Reported-by: Pascal de Bruijn <pmjdebruijn@pcode.nl>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=72690
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>