intel: Use GL_FRONT_AND_BACK for stencil clearing.

This comes from a radeon-rewrite fallback fix, but may also fix stencil
clear failure when the polygon winding mode is flipped.
(cherry picked from commit d866abeffc)

src/mesa/drivers/dri/intel/intel_clear.c

@@ -256,7 +256,8 @@ intel_clear_tris(GLcontext *ctx, GLbitfield mask)
       if (this_mask & BUFFER_BIT_STENCIL) {
 	 _mesa_Enable(GL_STENCIL_TEST);
 	 _mesa_StencilOp(GL_REPLACE, GL_REPLACE, GL_REPLACE);
-	 _mesa_StencilFuncSeparate(GL_FRONT, GL_ALWAYS, ctx->Stencil.Clear,
+	 _mesa_StencilFuncSeparate(GL_FRONT_AND_BACK, GL_ALWAYS,
+				   ctx->Stencil.Clear,
 				   ctx->Stencil.WriteMask[0]);
       } else {
 	 _mesa_Disable(GL_STENCIL_TEST);