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
(cherry picked from commit 5c5a46884899ea25cdf25545d6ab3d9a74eafa3a)
diff --git a/src/mesa/drivers/dri/i915/i915_tex_layout.c b/src/mesa/drivers/dri/i915/i915_tex_layout.c
index d44a2f4..7cc1c09 100644
--- a/src/mesa/drivers/dri/i915/i915_tex_layout.c
+++ b/src/mesa/drivers/dri/i915/i915_tex_layout.c
@@ -454,7 +454,10 @@
 {
    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);