[Night Color KCM] Don't show advanced controls for constant mode

Summary:
The user can only change the color temperature when the constant mode is
selected. Therefore, it doesn't make sense to display all irrelevant input
controls (e.g. latitude and longitude inputs, etc).

Test Plan:
{F7158822}
{F7158825}

Reviewers: #plasma, #vdg, GB_2, ngraham

Reviewed By: #vdg, GB_2, ngraham

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D22924