drm: Fix hdmi connector content type property docs

Apparently didn't get carefully checked.

Fixes: 50525c33 ("drm: content-type property for HDMI connector")
Cc: Hans Verkuil <hans.verkuil@cisco.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20180702091023.695-1-daniel.vetter@ffwll.ch