radeonsi: add support for cull distances. (v1.1)

This should be all that is required for cull distances to work
on radeonsi.

v1.1: whitespace cleanup, add docs fix clipdist_mask usage.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>

docs/relnotes/12.1.0.html

@@ -47,6 +47,7 @@ Note: some of the new features are only available with certain drivers.
 <li>OpenGL ES 3.1 on i965/hsw</li>
 <li>GL_ARB_ES3_1_compatibility on i965</li>
 <li>GL_ARB_clear_texture on r600, radeonsi</li>
+<li>GL_ARB_cull_distance on radeonsi</li>
 <li>GL_ARB_enhanced_layouts on i965</li>
 <li>GL_ARB_indirect_parameters on radeonsi</li>
 <li>GL_ARB_shader_draw_parameters on radeonsi</li>