i965/gen6: Issue direct depth stall and flush after depth clear

i965/gen6: Issue direct depth stall and flush after depth clear

instead of calling unconditionally brw_emit_mi_flush() which
does:

   brw_emit_pipe_control_flush(brw,
                                PIPE_CONTROL_DEPTH_CACHE_FLUSH |
                                PIPE_CONTROL_RENDER_TARGET_FLUSH |
                                PIPE_CONTROL_CS_STALL);

   brw_emit_pipe_control_flush(brw,
                                PIPE_CONTROL_TEXTURE_CACHE_INVALIDATE |
                                PIPE_CONTROL_CONST_CACHE_INVALIDATE);

Signed-off-by: Topi Pohjolainen <topi.pohjolainen@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>