g3dvl: Rework the decoder interface part 1/5

g3dvl: Rework the decoder interface part 1/5

First of all get ride of the decode_buffer structure, while still giving
the decoder the ability to organize it's buffers depending on the needs
of the state tracker.

Signed-off-by: Christian König <deathsimple@vodafone.de>
Reviewed-by: Younes Manton <younes.m@gmail.com>