The decoder context needs to know what engine it's associated with.
Nowadays, we have render, compute, blitter, even video engines being
used from the same driver. Rather than trying to have a single decoder
and thwacking the engine field back and forth between calls, we make
one per queue family, and stash a pointer in anv_queue for easy access.
(Ported from anv: 79caf8a44b682f731c0fd3b941f38cc1869efe2d)
[dancingmirrors: Adapted for elk.]
Kenneth Graunke