display: Unset grab_frame_action in meta_display_end_grab_op

Otherwise, if the user had moved the windows around using a mouse,
they can not be moved beyond the top edge of the primary monitor when
the tray needs to be brought up, . This is because
constrain_titlebar_visible looks at grab_frame_action and thinks that
the movement is being caused by the user trying to move the windows up
by grabbing the frame.

Fixes: https://bugzilla.gnome.org/677215