Clamp inverse transform coeffs after 1st/2nd pass

Add clamping to 16/18/20 bits after first pass of the inverse
transform for 8/10/12 bit source espectively, and clampaing to 16
bits after second stage of the inverse transform.

BUG=aomedia:30

Change-Id: I7a68334c77baecb359a4fb3b801fdfd9d5a62f29