r300g: rebuild winsys/pipe buffer handling and add buffer map

r300g: rebuild winsys/pipe buffer handling and add buffer map

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

It makes r300g use pb_bufmgr structs and adds usage of the cached
bufmgr for vertex/index buffers.

It also avoids mapping too often.

I'm not 100% sure this is perfect but it won't find its own bugs.

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