Use 8x8 DCT transform for I8X8 prediction mode

Apply 2D-DCT transform of dimension 8x8 to encode prediction
residuals of I8X8 mode.
Brought back block type 3 probability context model for 8x8 tokens,
which is used for the coefficients of Y blocks in I8x8 modes. The
coefficient costs estimate of I8X8 mode in rate-distortion is also
changed appropriately.
Performance results:
derf:   0.246
yt:     0.114
std-hd: 0.730
hd:     0.670

Change-Id: If1d970eeb4e1827c9f0d2c5b27d33089b347ea27