wayland/pointer: Check pointer presence at set focus call site

Make the caller of focus setting check whether there is a pointer to
update the focus state of. It makes it more obvious what to expect, as
the call would be a no-op in when no pointer is present.

Grabbing is still allowed without the presence of a pointer because it
is used by popups even on touch-only systems.

https://bugzilla.gnome.org/show_bug.cgi?id=771646