gdk/dnd: Don't use default display when getting cursor

gdk/dnd: Don't use default display when getting cursor

Always associate a drag context with a GdkDisplay and use that when
getting a cursor for a given action.

If we don't do this, dragging on a window that doesn't use the default
display will make us use cursors from the wrong display.

https://bugzilla.gnome.org/show_bug.cgi?id=765565