backend-drm: Correctly tear down the DRM backend

It seem that we skipped to put back in TEXT mode the tty, in case a DRM device node wasn't present at that time, or it isn't present at all. This orders the destroy part correctly as to handle that case as well.

As a side effect, as the tty will still be set to GRAPHICS mode we will require a manual change of the tty number, which might be not possible on all systems. Properly putting back the tty to TEXT mode should avoid that, and allows to re-use the same tty no in case the DRM device has been created at a later point in time.