radeon: allow driconf vblank settings with dri2

fixes:
https://bugs.freedesktop.org/show_bug.cgi?id=28771

NOTE: This is a candidate for the 7.8 branch.

src/mesa/drivers/dri/radeon/radeon_screen.c

@@ -1368,6 +1368,7 @@ radeonCreateScreen2(__DRIscreen *sPriv)
    screen->extensions[i++] = &driCopySubBufferExtension.base;
    screen->extensions[i++] = &driFrameTrackingExtension.base;
    screen->extensions[i++] = &driReadDrawableExtension;
+   screen->extensions[i++] = &dri2ConfigQueryExtension.base;
 
    if ( screen->irq != 0 ) {
        screen->extensions[i++] = &driSwapControlExtension.base;