gallium/dri2: Enable {GLX_ARB,EGL_KHR}_context_flush_control

Reviewed-and-tested-by: Nicolai Hähnle <nicolai.haehnle@amd.com>
Signed-off-by: Adam Jackson <ajax@redhat.com>

src/gallium/state_trackers/dri/dri2.c

@@ -1955,6 +1955,7 @@ static const __DRIextension *dri_screen_extensions[] = {
    &dri2FenceExtension.base,
    &dri2InteropExtension.base,
    &dri2NoErrorExtension.base,
+   &dri2FlushControlExtension.base,
    NULL
 };
 
@@ -1969,6 +1970,7 @@ static const __DRIextension *dri_robust_screen_extensions[] = {
    &dri2InteropExtension.base,
    &dri2Robustness.base,
    &dri2NoErrorExtension.base,
+   &dri2FlushControlExtension.base,
    NULL
 };