i965: add missing return in if statement

Re-add the "return false" that was removed in 0c02d7002d

It seems that something went wrong when merging the patch. The patch
sent to the mailing list does not directly match what was committed.
https://lists.freedesktop.org/archives/mesa-dev/2016-May/118198.html

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>

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

@@ -355,6 +355,7 @@ can_fast_copy_blit(struct brw_context *brw,
     */
    if (src_tr_mode == INTEL_MIPTREE_TRMODE_NONE &&
        dst_tr_mode == INTEL_MIPTREE_TRMODE_NONE)
+      return false;
 
    if (logic_op != GL_COPY)
       return false;