st/nine: Improve fallback when driver doesn't support user buffers.

st/nine: Improve fallback when driver doesn't support user buffers.

For now the path updated is only used by Amd drivers, but a later
patch will make it used by all drivers. Some drivers like llvmpipe
doesn't support the uploading of constants from user buffers, so improve
the path to work for all drivers

Inspired from the gl state tracker.

Signed-off-by: Axel Davy <axel.davy@ens.fr>