intrabc: adapt use_intrabc prob

First keyframe BD-RATE objective-1-fast:
   PSNR | PSNR Cb | PSNR Cr | PSNR HVS |    SSIM | MS SSIM | CIEDE 2000
-0.3705 | -0.3232 | -0.3812 |  -0.3782 |     N/A | -0.3412 |        N/A

First keyframe BD-RATE twitch-1:
   PSNR | PSNR Cb | PSNR Cr | PSNR HVS |    SSIM | MS SSIM | CIEDE 2000
-0.2479 | -0.2477 | -0.2467 |  -0.2567 | -0.2486 | -0.2508 |    -0.2487



Change-Id: Iea6c895c6fe9e9764887a8968f6e5330903969d3
diff --git a/av1/decoder/decodemv.c b/av1/decoder/decodemv.c
index 1cdbecb..b3ce86e 100644
--- a/av1/decoder/decodemv.c
+++ b/av1/decoder/decodemv.c
@@ -1017,7 +1017,7 @@
 
 #if CONFIG_INTRABC
   if (bsize >= BLOCK_8X8 && cm->allow_screen_content_tools) {
-    mbmi->use_intrabc = aom_read(r, INTRABC_PROB, ACCT_STR);
+    mbmi->use_intrabc = aom_read(r, ec_ctx->intrabc_prob, ACCT_STR);
     if (mbmi->use_intrabc) {
       mbmi->mode = mbmi->uv_mode = DC_PRED;
 #if CONFIG_DUAL_FILTER