Use priv_open_device() to open the dri device in glamor_dri3_open_client().

Fixes DRI3 with Xserver running as _x11 with xenodm.
close-on-exec is now default for priv_open_device().
ok kettenis@

xserver/glamor/glamor_egl.c

@@ -604,7 +604,11 @@ glamor_dri3_open_client(ClientPtr client,
     int fd;
     drm_magic_t magic;
 
+#ifndef __OpenBSD__
     fd = open(glamor_egl->device_path, O_RDWR|O_CLOEXEC);
+#else
+    fd = priv_open_device(glamor_egl->device_path);
+#endif
     if (fd < 0)
         return BadAlloc;