st/nine: Implement new buffer upload path

st/nine: Implement new buffer upload path

This new buffer upload path enables to lock
faster than the normal path when using
DISCARD/NOOVERWRITE.

v2: Diverse cleanups and fixes.
v3: Fix allocation size for 'lone' buffers and
add more debug info.
v4: Rewrite of the path to handle when DISCARD/NOOVERWRITE
is not used anymore. The resource content is copied to the
new resource used.
v5: flush for safety after unmap (not sure it is really required
here, but safer to flush).
v6: Do not use the path if persistent coherent mapping is unavailable.
Fix buffer creation flags.
v7: Do not flush since it is not needed.

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