xwayland: update_screen_size() takes a screen argument

update_screen_size() takes an xwl_output argument, mostly for historical
reasons, whereas it actually applies to a screen (as its name implies).

Reshuffle the code to take an xwl_screen instead, in preparation for
the geometry mode in Xwayland - No functional change.

Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>
Reviewed-by: Adam Jackson <ajax@redhat.com>