gallium: Expose a opaque winsys handle and functions on pipe_screen

gallium: Expose a opaque winsys handle and functions on pipe_screen

Instead of having these functions on a side interface like on
drm_api create a opaque winsys_handle that is to be passed down
into the winsys.

Currently the only thing ported to this new interface is drm_api,
and of that only the components that builds by default is ported.
All the drivers and any extra state trackers needs to be ported
before this can go into master.