In the pursuit of lowering driver overhead, it became clear that some
amount of redesign of how libdrm_freedreno constructs the submit ioctl
would be needed.  In particular, as the gallium driver is starting to
make heavier use of CP_SET_DRAW_STATE state groups/objects, the over-
head of tracking cmd buffers and relocs becomes too much.  And for
"streaming" state, which isn't ever reused (like uniform uploads) the
overhead of allocating/freeing ringbuffer[1] objects is too high.

This redesign makes two main changes:

 1) Introduces a fd_submit object for tracking bos and cmds table
    for the submit ioctl, making ringbuffer objects more light-
    weight.  This was previously done in the ringbuffer.  But we
    have many ringbuffer instances involved in a submit (gmem +
    draw + potentially 1000's of state-group rbs), and only need
    a single bos and cmds table.  (Reloc table is still per-rb)

    The submit is also a convenient place for a slab a...