r300g: rebuild screen/winsys interface

r300g: rebuild screen/winsys interface

This creates a cleaner winsys and drop the simple screen stuff.

It makes r300g use pb_bufmgr structs.

It also tries to avoid overheads from mapping too often.

v5: clean warnings

v6: break out of cache check on first buffer - since most likely
the first busy one implies all after it are busy.

v7: cleanup a bit

v8-merged: drop cman for now to just get all the interface changes in first.
rework to changes that happened upstream

Signed-off-by: Dave Airlie <airlied@redhat.com>