gdb/dwarf: pass dwarf2_cu instead of cutu_reader to two functions

These functions don't need to receive a cutu_reader, they only use it to
obtain the contained dwarf2_cu, so change them to accept a dwarf2_cu.
This helps reduce the creep of cutu_reader a little bit.

Change-Id: Iebb3c4697a4aec638b47423b3ac59077d4fa5090
Approved-By: Tom Tromey <tom@tromey.com>