st/mesa: expose EXT_disjoint_timer_query

same cap as ARB_timer_query, no changes needed, tested by piglit

docs/relnotes/18.3.0.html

@@ -55,6 +55,7 @@ Note: some of the new features are only available with certain drivers.
 <li>GL_AMD_gpu_shader_int64 on i965, nvc0, radeonsi.</li>
 <li>GL_AMD_multi_draw_indirect on all GL 4.x drivers.</li>
 <li>GL_AMD_query_buffer_object on i965, nvc0, r600, radeonsi.</li>
+<li>GL_EXT_disjoint_timer_query on radeonsi and most other Gallium drivers (ES extension)</li>
 <li>GL_EXT_vertex_attrib_64bit on i965, nvc0, radeonsi.</li>
 <li>GL_EXT_window_rectangles on radeonsi.</li>
 </ul>