Update ARF QP decision

Improve the coding performance for 4K 10 bit set (a1) by -0.53%
in RA mode.

Change-Id: I2d77b663d032702ff1ea87d34bf1898ffdce7ccd

av1/encoder/encoder_utils.c

diff --git a/av1/encoder/encoder_utils.c b/av1/encoder/encoder_utils.c
index fcc1087..6b53d74 100644
--- a/av1/encoder/encoder_utils.c
+++ b/av1/encoder/encoder_utils.c
@@ -558,8 +558,8 @@
       const double qratio_grad =
           cpi->ppi->p_rc.baseline_gf_interval > 20 ? 0.2 : 0.3;
       const double qstep_ratio =
-          0.2 +
-          (1.0 - (double)cpi->rc.active_worst_quality / MAXQ) * qratio_grad;
+          0.2 + (1.0 - pow((double)cpi->rc.active_worst_quality / MAXQ, 2.0)) *
+                    qratio_grad;
       *q = av1_get_q_index_from_qstep_ratio(
           cpi->rc.active_worst_quality, qstep_ratio, cm->seq_params->bit_depth);
       *top_index = *bottom_index = *q;