drm/TODO: Clarify DRM_DEV_* conversion

Not everyone agrees this is the best thing, so make it really clear
that maintainers need to be asked first, then the conversion. We've
had a few newbies that did this the other way round, got their patches
rejected, which isn't the best newbie experience.

Cc: Sean Paul <seanpaul@chromium.org>
Acked-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171030131536.11654-1-daniel.vetter@ffwll.ch