optimize 8x8 fdct rounding for accuracy

The commit added a final rounding choice for 8x8 forward dct to get
rid of a sign bias at DC position and improve the accuracry in term
of round trip error for 8x8 fDCT/iDCT.

This commit also enabled forward 8x8 dct test.

Change-Id: Ib67f99b0a24d513e230c7812bc04569d472fdc50