freedreno/msm: use hashtable to track submit.cmds table

With streaming stateobjs to upload uniforms, the submit.cmds table gets
much larger, and iterating over it for each ring to ring reloc starts
getting expensive.

TODO if we have flag to pass when constructing parent rb, we could
avoid dynamically allocating this and bo_table in get_cmd() or bo2idx

Signed-off-by: Rob Clark <robclark@freedesktop.org>