lavapipe: rewrite cmdbufs to always do descriptor binds/pushes first

lavapipe: rewrite cmdbufs to always do descriptor binds/pushes first

llvmpipe has some crazy dependency ordering with constant buffers that
I don't want to mess with, so instead this ensures that descriptors are
always updated as early in the command stream as possible, avoiding any
use-after-free scenarios which might result from lavapipe freeing memory
in a still-bound descriptor that's pending an update

Reviewed-by: Dave Airlie <airlied@redhat.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/9692>