nine/pipe: optimize convert_dsa_state a bit
nine/buffer: use PIPE_USAGE_STREAM for D3DUSAGE_DYNAMIC

Which is the usual use case and also what wine picks.
We have have to refine that at some point, but seems to work nicely
for SC2.
nine/pipe: scale DEPTHBIAS value

Fixes zfighting of pebbles on the floor in Skyrim.