seat-native: Fix has_touchscreen becoming FALSE when a non touchscreen device gets added

Unconditionally setting has_touchscreen to check_touch_mode
when a new device gets added leads to has_touchscreen becoming
false when during runtime e.g. an USB keyboard gets plugged in.

Fix this by setting has_touchscreen to TRUE when check_touch_mode
is TRUE and leaving it alone otherwise.

https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1506
(cherry picked from commit 6c240dc8)

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1610>