Fix vmaf model initialization error when not set to tune=vmaf

BUG=aomedia:3021

Change-Id: I70431be1fab71c59b66d37813637170f5d78864b

av1/av1_cx_iface.c

diff --git a/av1/av1_cx_iface.c b/av1/av1_cx_iface.c
index 0787c49..ab7a403 100644
--- a/av1/av1_cx_iface.c
+++ b/av1/av1_cx_iface.c
@@ -2492,8 +2492,11 @@
   }
 
 #if CONFIG_TUNE_VMAF
-  aom_init_vmaf_model(&cpi->vmaf_info.vmaf_model,
-                      cpi->oxcf.tune_cfg.vmaf_model_path);
+  if (ctx->extra_cfg.tuning >= AOM_TUNE_VMAF_WITH_PREPROCESSING &&
+      ctx->extra_cfg.tuning <= AOM_TUNE_VMAF_NEG_MAX_GAIN) {
+    aom_init_vmaf_model(&cpi->vmaf_info.vmaf_model,
+                        cpi->oxcf.tune_cfg.vmaf_model_path);
+  }
 #endif
 
   // Handle fixed keyframe intervals