drm: Remove dirty property from docs

We removed it in

commit 6ab10b76
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Fri Aug 12 22:48:45 2016 +0200

    drm/kms: Nuke dirty_info property

Reviewed-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: http://patchwork.freedesktop.org/patch/msgid/1474448370-32227-7-git-send-email-daniel.vetter@ffwll.ch