kconfig: qconf: move conf_read() before drawing tree pain
The constructor of ConfigMainWindow() calls show*View(), which needs
to calculate symbol values. conf_read() must be called before that.
Fixes: 060e05c3b422 ("kconfig: qconf: remove initial call to conf_changed()")
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>