xv: Move the DDX XV screen private allocation into the DDXes.

XV was going against convention by having the core infrastructure
allocate the private on behalf of the DDX.  I was interested in this
because I was trying to make multiple pieces of DDX be able to
allocate adaptors, and that wasn't going to work if DDX-specific code
was hung off of a single global screen private.

Signed-off-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Keith Packard <keithp@keithp.com>