frameencoder: do not allocate noise reduction struct unless NR is enabled

This fixes output mismatch between asm / no-asm for 4:4:4 encodes.  Why? I am
not entirely sure yet, but this is an obvious bug fix.

--HG--
branch : stable
extra : rebase_source : 9a3ffc24b47c1c4b1532f26168c14ba7e9cab5ed
search: cleanup variable names and lossless comments
search: nit
Merge with stable
cleanup: remove reduce condition check
fix single thread encode issue