Use gdbpy_enter_varobj in more of varobj.c

Use gdbpy_enter_varobj in more of varobj.c

This converts most of the remaining functions in varobj.c to use
gdbpy_enter_varobj.

2017-01-10  Tom Tromey  <tom@tromey.com>

	* varobj.c (varobj_get_display_hint)
	(dynamic_varobj_has_child_method, install_new_value_visualizer)
	(varobj_set_visualizer, free_variable): Use
	gdbpy_enter_varobj.