Avoid gf/altref boost in pick_q under svc cbr mode.

Change-Id: Ib719a9f74924718715592e0238bc391ae239d026

vp9/encoder/vp9_ratectrl.c

diff --git a/vp9/encoder/vp9_ratectrl.c b/vp9/encoder/vp9_ratectrl.c
index 89aa821..8430e4b 100644
--- a/vp9/encoder/vp9_ratectrl.c
+++ b/vp9/encoder/vp9_ratectrl.c
@@ -587,6 +587,7 @@
                                                    q_adj_factor);
     }
   } else if (!rc->is_src_frame_alt_ref &&
+             !cpi->use_svc &&
              (cpi->refresh_golden_frame || cpi->refresh_alt_ref_frame)) {
     // Use the lower of active_worst_quality and recent
     // average Q as basis for GF/ARF best Q limit unless last frame was