This just adds the structures and interfaces required for adding/deleteing
gpu screens at the DDX level. The platform probe can pass a new flag
to the driver, so they can call xf86AllocateScreen and pass back the new
gpu screen flag.

It also calls the gpu screens preinit and screeninit routines at
startup.

v2: fix delete screen use after free.

v3: split out pScrn into separate patch

Reviewed-by: Keith Packard <keithp@keithp.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>