Refactor quantization C code.

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
diff --git a/aom_dsp/aom_dsp_common.h b/aom_dsp/aom_dsp_common.h
index 5b10432..a3cea56 100644
--- a/aom_dsp/aom_dsp_common.h
+++ b/aom_dsp/aom_dsp_common.h
@@ -52,10 +52,9 @@
 #define UNLIKELY(v) (v)
 #endif
 
-#if CONFIG_AOM_QM
 typedef uint16_t qm_val_t;
 #define AOM_QM_BITS 5
-#endif
+
 #if CONFIG_HIGHBITDEPTH
 // Note:
 // tran_low_t  is the datatype used for final transform coefficients.