Notify the driver when Scratch Pixmaps are released

[Chris Wilson]

If the driver uses the Pixmap to track GPU objects associated with
transient data structures like scratch Pixmaps, then it needs to be
notified when the resource is released. Currently, that notification is
only performed when the scratch Pixmap is reused - too late. One
solution, is to add a miModifyPixmapHeader() into FreeScratchPixmaps
to provide the driver notification. An alternative is to drop the
malloc caching behaviour and free the scratch Pixmap everytime; the
ScratchPixmapHeader then just becomes an ordinary Pixmap like any other,
just simply with a bounded lifetime.

The other patches are related changes upon reviewing scratch Pixmap
usage within the core server.

Kindly review, thanks.
-Chris