drm/print: underscore prefix functions that should be private to print

We don't want people calling the functions directly. No functional
changes.

Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Acked-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Acked-by: Sean Paul <sean@poorly.run>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/6b236ed4d2e6d2987eaaeb9cb737f9c3699281cc.1572258936.git.jani.nikula@intel.com