Remove redundant mode update in sub8x8 decoding

Remove redundant mode update in sub8x8 decoding

The probability model used to code prediction mode is conditioned
on the immediate above and left 8x8 blocks' prediction modes. When
the above/left block is coded in sub8x8 mode, we use the prediction
mode of the bottom-right sub8x8 block as the reference to generate
the context.

This commit moves the update of mbmi.mode out of the sub8x8 decoding
loop, hence removing redundant update steps and keeping the bottom-
right block's mode for the decoding process of next blocks.

Change-Id: I1e8d749684d201c1a1151697621efa5d569218b6