GLX/DRI2: Do not expose INTEL_swap_event without swap control

GLX/DRI2: Do not expose INTEL_swap_event without swap control

Swap events depent on the implementation of ScheduleSwap. By
unconditionally enabling GLX_INTEL_swap_event we're breaking
the system with drivers that don't support it because the apps
are forever stuck waiting for an event that will never be
delivered. So lets enable the extension only if the hooks it
depends on are actually there.

Signed-off-by: Zack Rusin <zackr@vmware.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Signed-off-by: Keith Packard <keithp@keithp.com>