Scrollview - Don't fill the parent with the view

Summary:
If the style sets a padding this is not respected if we set `anchors.fill: parent`.

BUG: 407643

Test Plan:
Before:
{F7119700}
After:
{F7119699}

Reviewers: mart, ngraham

Reviewed By: mart, ngraham

Subscribers: ngraham, filipf, kde-frameworks-devel

Tags: #frameworks

Differential Revision: https://phabricator.kde.org/D22827