i915: Only use the new 945 cube layout for compressed textures.

The docs actually explain this, but not in a terribly clear manner.
This nearly fixes the piglit cubemap testcase, except that something's
going wrong with the nearest filtering at 2x2 sizes in the testcase.
Looks good by visual inspection, though.

Bug #21692

src/mesa/drivers/dri/i915/i915_tex_layout.c

@@ -454,7 +454,10 @@ i945_miptree_layout(struct intel_context *intel, struct intel_mipmap_tree * mt)
 {
    switch (mt->target) {
    case GL_TEXTURE_CUBE_MAP:
-      i945_miptree_layout_cube(intel, mt);
+      if (mt->compressed)
+	 i945_miptree_layout_cube(intel, mt);
+      else
+	 i915_miptree_layout_cube(intel, mt);
       break;
    case GL_TEXTURE_3D:
       i945_miptree_layout_3d(intel, mt);