drmdevice: convert the tabbed output into a tree

Making the output a little bit easier to parse by human beings.

v2: Add extra whitespace (Eric)

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Tested-by: Robert Foss <robert.foss@collabora.com> (v1)
Reviewed-by: Robert Foss <robert.foss@collabora.com> (v1)
Reviewed-by: Eric Engestrom <eric@engestrom.ch> (v1)