nv30: reset the stencil mask when fast-clearing

Apparently the stencil mask applies to clears on nv30/nv40. Reset it to
0xff before doing a stencil clear. This fixes gl-1.0-readpixsanity and
a number of other piglit tests.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
diff --git a/src/gallium/drivers/nouveau/nv30/nv30_clear.c b/src/gallium/drivers/nouveau/nv30/nv30_clear.c
index 118cac7..c8fa38e 100644
--- a/src/gallium/drivers/nouveau/nv30/nv30_clear.c
+++ b/src/gallium/drivers/nouveau/nv30/nv30_clear.c
@@ -73,8 +73,13 @@
       zeta = pack_zeta(fb->zsbuf->format, depth, stencil);
       if (buffers & PIPE_CLEAR_DEPTH)
          mode |= NV30_3D_CLEAR_BUFFERS_DEPTH;
-      if (buffers & PIPE_CLEAR_STENCIL)
+      if (buffers & PIPE_CLEAR_STENCIL) {
          mode |= NV30_3D_CLEAR_BUFFERS_STENCIL;
+         BEGIN_NV04(push, NV30_3D(STENCIL_ENABLE(0)), 2);
+         PUSH_DATA (push, 0);
+         PUSH_DATA (push, 0x000000ff);
+         nv30->dirty |= NV30_NEW_ZSA;
+      }
    }
 
    /*XXX: wtf? fixes clears sometimes not clearing on nv3x... */