Can be expanded up on to include all sorts of things (HDMI infoframe
data, more DP status, etc).  Should be useful for bug reports to get a
baseline on the display config and info.

v2: use seq_putc (Rodrigo)
    describe mode field names (Rodrigo)

Reviewed-by: Rodrigo Vivi <rodrigo.vivi@gmail.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>