DRI2: HACK: no GLX_INTEL_swap_event if no ScheduleSwap

DRI2: HACK: no GLX_INTEL_swap_event if no ScheduleSwap

If ddx does not support swap, don't advertise it.  This is a hack to
work around current xservers which advertise this extension even when it
is clearly not supported.  When:

http://lists.x.org/archives/xorg-devel/2013-February/035449.html

is merged in upstream xserver and makes it's way into most distros then
this hack can be removed.  In the mean time, it is required to allow
gnome-shell/clutter/etc to work properly with a DDX driver which does
not support ScheduleSwap.

Signed-off-by: Rob Clark <robdclark@gmail.com>