Moved dequant idct into common

These functions are now used by the encoder.
This is WIP with the goal of creating a common idct/add for
the encoder and decoder.  A boost of 1.8% was seen for
the HD rt test clip used.

[Tero] Added needed changes to ARM side.

Change-Id: Ibbb8000be09034203d7adffc457d3c3f8b06a5bf