i915: Silence unused parameter warning in intel_miptree_create_layout

i915: Silence unused parameter warning in intel_miptree_create_layout

The for_bo parameter of intel_miptree_create_layout appears to be unused
since 27eedca when Eric removed some Gen5 code (after the i915 and i965
drivers parted ways).

intel_mipmap_tree.c: In function 'old_intel_miptree_create_layout':
intel_mipmap_tree.c:77:35: warning: unused parameter 'for_bo' [-Wunused-parameter]
                             bool for_bo)
                                   ^

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