This commit makes zcoeff_blk cache the case where the entire block
is quantized to be zero (without applying zero-forcing) in the rate-
distortion optimization loop, and skip the forward DCT, quantization,
inverse DCT, and reconstruction process in the encode_block stage.

It now works for all the block sizes, including sub8x8 blocks.

Change-Id: I5ae60a9c436ba3637d11666733554bec4580ef98