opus_pvq: merge band encoding and decoding into one function

Most code between the 2 functions was duplicated which made keeping
both in sync difficult.

This also fixes some discovered issues with encoding (incorrect
TF switching buffers) and reduces stack usage (reuse the already
allocated CeltFrame->scratch buffer for the quantized coefficients).
Signed-off-by: Rostislav Pehlivanov <atomnuker@gmail.com>