ilo: rework winsys bo reloc functions

ilo: rework winsys bo reloc functions

Rename

  intel_bo_emit_reloc() to intel_bo_add_reloc(),
  intel_bo_clear_relocs() to intel_bo_truncate_relocs(), and
  intel_bo_references() to intel_bo_has_reloc().

Besides, we need intel_bo_get_offset() only to get the presumed offset afer
adding a reloc entry.  Remove the function and make intel_bo_add_reloc()
return the presumed offset.  While at it, switch to gem_bo->offset64 from
gem_bo->offset.