Protect new metric computation with use_highbitdepth flag

The computation of new metrics is not supported yet in highbitdepth
mode. This commit adds protection to make sure the computation is
done only when highbitdepth is not on. This protection shall be
revised when support of highbitdpeth computation is added.

This resolves the encoder crash when configured with both
--enable-internal-stats
--enable-vp9-highbitdepth

Change-Id: Id9f4bcc4fa26d9ca0e9eabade83f3f88a5b212e6