i915, i965: Silence unused parameter warnings in intel_mipmap_tree.h

i915, i965: Silence unused parameter warnings in intel_mipmap_tree.h

These only occurred in release builds, but they occurred in every file
that included intel_mipmap_tree.h.  Lots of spam. :(

intel_mipmap_tree.h: In function 'intel_miptree_check_level_layer':
intel_mipmap_tree.h:595:59: warning: unused parameter 'mt' [-Wunused-parameter]
 intel_miptree_check_level_layer(struct intel_mipmap_tree *mt,
                                                           ^
intel_mipmap_tree.h:596:42: warning: unused parameter 'level' [-Wunused-parameter]
                                 uint32_t level,
                                          ^
intel_mipmap_tree.h:597:42: warning: unused parameter 'layer' [-Wunused-parameter]
                                 uint32_t layer)
                                          ^

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com>