Prevent negative cost for highbitdepth

Adds proper scaling for highbitdepth in a rdopt cost.

Change-Id: I066694799a7f491b830945ef1c66eb202071c355

vp9/encoder/vp9_rdopt.c

diff --git a/vp9/encoder/vp9_rdopt.c b/vp9/encoder/vp9_rdopt.c
index b14ff08..0ae0d57 100644
--- a/vp9/encoder/vp9_rdopt.c
+++ b/vp9/encoder/vp9_rdopt.c
@@ -491,7 +491,9 @@
       if (x->plane[plane].eobs[block]) {
         int64_t dc_correct = coeff[0] * coeff[0] -
             (coeff[0] - dqcoeff[0]) * (coeff[0] - dqcoeff[0]);
-
+#if CONFIG_VP9_HIGHBITDEPTH
+        dc_correct >>= ((xd->bd - 8) * 2);
+#endif
         if (tx_size != TX_32X32)
           dc_correct >>= 2;