It was added way back together with the dirty_fb ioctl, but neither
generic xfree86-modesetting nor the vmware driver use it. Everyone is
supposed to just unconditionally call the dirtyfb when they do
frontbuffer rendering.

And since unused uabi is bad uabi (there's reasons we require open
source userspace for everything) let's nuke this.

For reference see

commit 884840aa
Author: Jakob Bornecrantz <jakob@vmware.com>
Date:   Thu Dec 3 23:25:47 2009 +0000

    drm: Add dirty ioctl and property

Cc: Jakob Bornecrantz <jakob@vmware.com>
Cc: Dave Airlie <airlied@redhat.com>
Cc: Sinclair Yeh <syeh@vmware.com>
Cc: Thomas Hellstrom <thellstrom@vmware.com>
Acked-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1471034937-651-9-git-send-email-daniel.vetter@ffwll.ch