r600g: remove unused flag have_depth_fb

This is a leftover from:

commit fe1fd675565231b49d3ac53d0b4bec39d8bc6781
Author: Marek Olšák <maraeo@gmail.com>
Date:   Sun Jul 8 03:10:37 2012 +0200

    r600g: don't flush depth textures set as colorbuffers
4 files changed