[platforms/drm] Determine the supported transformation on a plane

Summary:
The idea behind getting the supported transformations is to tell KScreen
which transformations are actually supported and thus not even allow the
user select a setting which is not supported.

Reviewers: #kwin, #plasma, subdiff

Subscribers: plasma-devel, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D8645