This commit de-duplicates C reference quantization code
and unifies quantization matrix (QM) and non-QM code
paths when there is no SIMD.

The reorganisation also will facilitate re-using SIMD quant
functions for QM when the matrix is flat, as is the
default when AOM_QM is enabled.

Change-Id: Idbfdac9eb9a31adcffe734aac1877d58b86fab77