r600: don't set an index_bias for indirect draw calls
The indirect draw call already encodes the index bias so that no additional encoding in the hardware is needed in this case. This fixes a regression with a number of tests from dEQP-GLES31.functional.draw_indirect.random.* Fixes: c6c532fa "gallium/u_vbuf: use updated pipe_draw_start_count while using draw_vbo" Signed-off-by:Gert Wollny <gert.wollny@collabora.com> Part-of: <mesa/mesa!9877> (cherry picked from commit acdf1a12)