wsi/x11: Fetch and discard the SYNC extension info

wsi/x11: Fetch and discard the SYNC extension info

Since we're not checking for this, xcb has to do it for us the first
time we call xcb_sync_destroy_fence, which puts a blocking round-trip in
the swapchain destroy path for no reason. Check for the extension so we
have the extension's opcode cached when we need it.

Reviewed-by: Michel Dänzer <mdaenzer@redhat.com>
Reviewed-By: Mike Blumenkrantz <michael.blumenkrantz@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/13339>