i965: Ensure that texture validation is skipped for immutable textures.

If we were to relayout the miptree, we'd break any views that are
sharing it.

(Simplified based on suggestions from Eric)

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Acked-by: Eric Anholt <eric@anholt.net>

src/mesa/drivers/dri/i965/intel_tex_validate.c

@@ -98,6 +98,11 @@ intel_finalize_mipmap_tree(struct brw_context *brw, GLuint unit)
       return true;
    }
 
+   /* Immutable textures should not get this far -- they should have been
+    * created in a validated state, and nothing can invalidate them.
+    */
+   assert(!tObj->Immutable);
+
    firstImage = intel_texture_image(tObj->Image[0][tObj->BaseLevel]);
 
    /* Check tree can hold all active levels.  Check tree matches