Fix prob of disabling mmi on loongson3 variant.

If 'mips_arch_variant=loongson3' is setted without 'mips_use_mmi=true',
undefined reference of MMI functions will be reported.

Change-Id: I2e91d63e1fa2d6c0db907aaab978227c87c991ac
Reviewed-on: https://chromium-review.googlesource.com/c/libyuv/libyuv/+/2129285
Reviewed-by: Frank Barchard <fbarchard@chromium.org>
Commit-Queue: Frank Barchard <fbarchard@chromium.org>

BUILD.gn

@@ -176,6 +176,9 @@ static_library("libyuv_internal") {
       "-ffp-contract=fast",  # Enable fma vectorization for NEON.
     ]
   }
+  if (!libyuv_use_mmi) {
+    defines += [ "LIBYUV_DISABLE_MMI" ]
+  }
 }
 
 if (libyuv_use_neon) {