drm/doc: move printf helpers out of drmP.h

And document them lightly. Unfortunately kernel-doc isn't the most
awesome for documenting #defines that don't look like functions, it
makes functions out of them :-/

Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170524145212.27837-2-daniel.vetter@ffwll.ch
Link: http://patchwork.freedesktop.org/patch/msgid/20170524145212.27837-3-daniel.vetter@ffwll.ch