drm/i915: add a display info file to debugfs v2

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>