powerprofileconfig: Use m_profileCombo as the watcher's parent

`m_profileCombo` could be an invalid pointer, because when clicking an
item, `QAbstractItemView::clicked` and `QAbstractItemView::activated` in
systemsettings both will be emitted, and `IconMode::changeModule` will be
called twice, and `closeModules()` will free `m_profileCombo` in the
second call, which will crash systemsettings.

To fix the crash, set the watcher's parent to `m_profileCombo`, so when
`m_profileCombo` is destroyed, the watcher is also destroyed and the
connection is disconnected to prevent modification to the old `m_profileCombo`.

CCBUG: 443858


(cherry picked from commit 9b8a9dfd)