radeonsi: add fine derivate control (v2.1)

This adds support for fine derivatives and enables
ARB_derivative_control on radeonsi.

(just fell out of my working out interpolation)

v2: cleanup some bits, write a comment
v2.1: take Michel's comment from the mailing list

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

docs/GL3.txt

@@ -191,7 +191,7 @@ GL 4.5, GLSL 4.50:
   GL_ARB_clip_control                                  DONE (i965, nv50, nvc0, r600, radeonsi, llvmpipe, softpipe)
   GL_ARB_conditional_render_inverted                   DONE (i965, nv50, nvc0, llvmpipe, softpipe)
   GL_ARB_cull_distance                                 in progress (Tobias)
-  GL_ARB_derivative_control                            DONE (i965, nv50, nvc0, r600)
+  GL_ARB_derivative_control                            DONE (i965, nv50, nvc0, r600, radeonsi)
   GL_ARB_direct_state_access                           DONE (all drivers)
   GL_ARB_get_texture_sub_image                         DONE (all drivers)
   GL_ARB_shader_texture_image_samples                  not started

docs/relnotes/10.7.0.html

@@ -45,6 +45,7 @@ Note: some of the new features are only available with certain drivers.
 
 <ul>
 <li>GL_AMD_vertex_shader_viewport_index on radeonsi</li>
+<li>GL_ARB_derivative_control on radeonsi</li>
 <li>GL_ARB_fragment_layer_viewport on radeonsi</li>
 <li>GL_ARB_framebuffer_no_attachments on i965</li>
 <li>GL_ARB_get_texture_sub_image for all drivers</li>