compositor: Use pointer_cursor_surface_configure() in wl_pointer.set_cursor

compositor: Use pointer_cursor_surface_configure() in wl_pointer.set_cursor

This way we map the surface if it currently isn't mapped and avoid
duplicating some of the code already in pointer_cursor_surface_configure().
Without this, the cursor code relied on a wl_surface.attach() to show the
new pointer surface.  If we're not changing the cursor buffer, we don't
get that, but we still need to map the cursor.