i965: Return BRW_DEPTHBUFFER_D32_FLOAT as the null-depthbuffer format.

Fixes many crashes on Ivybridge due to upload_sf_state calling
brw_depthbuffer_format without an actual depth buffer.  This was a
recent regression on master.

+3992 piglits on Ivybridge.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Eric Anholt <eric@anholt.net>

src/mesa/drivers/dri/i965/brw_misc_state.c

@@ -212,6 +212,9 @@ brw_depthbuffer_format(struct brw_context *brw)
       drb = srb;
    }
 
+   if (!drb)
+      return BRW_DEPTHFORMAT_D32_FLOAT;
+
    switch (drb->Base.Format) {
    case MESA_FORMAT_Z16:
       return BRW_DEPTHFORMAT_D16_UNORM;