st/nine: Optimize ColorFill

When we lock the whole surface to overwrite it, we can use DISCARD.

Signed-off-by: Axel Davy <axel.davy@ens.fr>
diff --git a/src/gallium/state_trackers/nine/device9.c b/src/gallium/state_trackers/nine/device9.c
index 28442eb..66bfc8d 100644
--- a/src/gallium/state_trackers/nine/device9.c
+++ b/src/gallium/state_trackers/nine/device9.c
@@ -1800,7 +1800,7 @@
         union util_color uc;
         HRESULT hr;
         /* XXX: lock pRect and fix util_fill_rect */
-        hr = NineSurface9_LockRect(surf, &lock, NULL, 0);
+        hr = NineSurface9_LockRect(surf, &lock, NULL, pRect ? 0 : D3DLOCK_DISCARD);
         if (FAILED(hr))
             return hr;
         util_pack_color_ub(color >> 16, color >> 8, color >> 0, color >> 24,