docs: document Gallium SSE codegen for XPD bug fix

docs/relnotes-7.7.1.html

@@ -42,6 +42,7 @@ tbd
 <li>Fixed broken blending to multiple color buffers in swrast driver.
 <li>Allocate constants more tightly in GL_ARB_vertex/fragment parser.
 <li>Fixed mipmap generation bug caused by invalid viewport state.
+<li>Gallium SSE codegen for XPD didn't always work.
 </ul>