ivi-shell: handle subsurfaces for mouse/touch activation

The surface of the focus_view may be a subsurface. So get the corresponding main
surface first.

Without this get_ivi_shell_surface(), will cast committed_private (which is a
weston_subsurface object) to ivi_shell_surface. Weston crashes shortly
afterwards.

Signed-off-by: Michael Olbrich <m.olbrich@pengutronix.de>