migrate to new helper API.
This just wraps all the obvious uses of xf86Screens[pScreen->myNum],
and should be fairly simple to review.
v2: remove commented out lines.
Reviewed-by: Aaron Plattner <aplattner@nvidia.com>
Reviewed-by: Keith Packard <keithp@keithp.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>