pan/midgard: Add mir_choose_instruction stub

pan/midgard: Add mir_choose_instruction stub

In the future, this routine will implement the core scheduling logic to
decide which instruction out of the worklist will be scheduled next, in
a way that minimizes cycle count and register pressure.

In the present, we are more interested in replicating in-order
scheduling with the much-more-powerful out-of-order model. So rather
than discriminating by a register pressure estimate, we simply choose
the latest possible instruction in the worklist.

Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>