st/nine: Implement Fast path for dynamic buffers and csmt

st/nine: Implement Fast path for dynamic buffers and csmt

Use the secondary pipe for DISCARD/NOOVERWRITE, which
avoids stalling to get the pipe from the worker thread.

v2: flush at unmap. This is required for example if
the driver does hidden draw calls or copies. In the case
of unsynchronized it is probably not required, but
it is more safe.

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