glx/kopper: don't call glFlush from swapbuffers

this is redundant and leads to extra glthread syncs

Reviewed-by: Adam Jackson <ajax@redhat.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/36887>

src/glx/drisw_glx.c

@@ -558,13 +558,13 @@ driswSwapBuffers(__GLXDRIdrawable * pdraw,
    (void) divisor;
    (void) remainder;
 
+   if (psc->kopper)
+       return kopperSwapBuffers(pdraw->dri_drawable, flush ? __DRI2_FLUSH_CONTEXT : 0);
+
    if (flush) {
       CALL_Flush(GET_DISPATCH(), ());
    }
 
-   if (psc->kopper)
-       return kopperSwapBuffers(pdraw->dri_drawable, flush ? __DRI2_FLUSH_CONTEXT : 0);
-
    driSwapBuffers(pdraw->dri_drawable);
 
    return 0;