intel-decode: Fix gen6 HIER_DEPTH_BUFFER decoding

It accidentally used the cmd id for the gen7 command and had an
outdated lenght field. Spotted while trying to make sense of an ivb
error_state from mesa 7.11 ...

Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>