[platforms/drm] Consider DSI connector type to be internal display

Summary:
Lot of mobile devices, as well as tablet devices use the DSI connector,
for internal screens. This is more common in ARM devices, so consider
them as internal display as well.

Test Plan: checked on the device with DSI screen, it was considered internal

Reviewers: KWin, romangg

Reviewed By: KWin, romangg

Subscribers: romangg, kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D24209