[kcms/nightcolour] Reset needsSave after loading initial QML properties

Sequence of events is:

1. KCModule::showEvent()
2. this queues up a load and queues up a KCModule::changed(false)

3. during load ConfigModule::setNeedsSave(true) is called we set

d->_needsSave to true

4. we emit ConfigModule::changed(true) which we proxy through to

KCModule::changed(true)

5. we then process the queued KCModule::setChanged(false) from the

earlier KCModule::showEvent

6. so we disable the button

7. any subsequent changes in the KCM will call

ConfigModule::setNeedsSave(true)
but this now matches d->_needsSave so it no-ops. Even though KCModule is
out of sync.

Systemsettings only knows what KCModule signals, not ConfigModule.

This patch resets ConfigModule::d->_needsSave after step 3

See also https://phabricator.kde.org/D27384#611241

BUG: 411584