Using pd->dqcoeff instead of pd->qcoeff in the decoder.

It is more logical to use dqcoeff buffer to put there *dequantized*
transform coefficients (inside inverse_transform_block and
decode_coefs functions). Dequantization happens inside WRITE_COEF_CONTINUE
macro.

qcoeff buffer should be only used in the encoder for *quantized*
transform coefficients.

Change-Id: Ifd54bef272bbf5311ced6669c4f1079f998af5d7