docs: document a mipmap generation bug fix

docs/relnotes-7.7.1.html

@@ -41,6 +41,7 @@ tbd
 <li>Assorted fixes to VMware SVGA gallium driver.
 <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.
 </ul>