st/nine: Finish if nooverwrite after normal mapping

st/nine: Finish if nooverwrite after normal mapping

d3d's nooverwrite and gallium's unsynchronized
have different semantics.
Indeed nooverwrite says the applications won't
write to locations needed by previous draws,
which is less strong than unsynchronized which
won't synchronize previous writes.

Thus in case app is locking without discard/nooverwrite,
then using nooverwrite, we need to add a
synchronization.

Fixes: https://github.com/iXit/wine-nine-standalone/issues/29

Signed-off-by: Axel Davy <davyaxel0@gmail.com>