[platforms/drm] Rework ScopedDrmPointer

Summary:
Currently there is no any good reason for keeping ScopedDrmPointer;
providing our own deleter for QScopedPointer would make more sense.

Given that we already have type that acts as a scoped pointer for drm
objects we can improve it a bit and make simpler, e.g.

DrmScopedPointer<drmModeConnector> connector;

is much simpler than

ScopedDrmPointer<_drmModeConnector, &drmModeFreeConnector> connector;

Reviewers: KWin, davidedmundson

Reviewed By: KWin, davidedmundson

Subscribers: apol, kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D19905