intel: Remove pointless boolean return value from *_miptree_layout.

intel: Remove pointless boolean return value from *_miptree_layout.

i915_miptree_layout, i945_miptree_layout, and brw_miptree_layout always
just return GL_TRUE, so there's really no point to it.  Change them to
void functions and remove the (dead) error checking code.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>