drm: drop unused drm_display_mode.private

drm_display_mode.private was only referenced in one place where
is was copied but never assigned.

Drop the copy and drop the field in drm_display_mode.
Adjust the comment of private_flags as is referred to the comment for
private.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Sean Paul <seanpaul@chromium.org>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200215183503.GA17310@ravnborg.org