i965: Use ctx->Stencil._WriteEnabled in DEPTH_STENCIL_STATE.

This is the same computation as the _WriteEnabled flag, so we may as
well use it.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Paul Berry <stereotype441@gmail.com>

src/mesa/drivers/dri/i965/gen6_depthstencil.c

@@ -74,11 +74,7 @@ gen6_upload_depth_stencil_state(struct brw_context *brw)
 	 ds->ds1.bf_stencil_test_mask = ctx->Stencil.ValueMask[back];
       }
 
-      /* Not really sure about this:
-       */
-      if (ctx->Stencil.WriteMask[0] ||
-	  (ctx->Stencil._TestTwoSide && ctx->Stencil.WriteMask[back]))
-	 ds->ds0.stencil_write_enable = 1;
+      ds->ds0.stencil_write_enable = ctx->Stencil._WriteEnabled;
    }
 
    /* _NEW_DEPTH */