drm/printer: add debug printer

Useful for dumping lots of data into dmesg, e.g. drm_mm.

v2: Fixup export_symbol line, I misplaced a hunk (Chris).

Cc: Rob Clark <robdclark@gmail.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1482943330-11592-1-git-send-email-daniel.vetter@ffwll.ch

drivers/gpu/drm/drm_print.c

@@ -40,6 +40,12 @@ void __drm_printfn_info(struct drm_printer *p, struct va_format *vaf)
 }
 EXPORT_SYMBOL(__drm_printfn_info);
 
+void __drm_printfn_debug(struct drm_printer *p, struct va_format *vaf)
+{
+	pr_debug("%s %pV", p->prefix, vaf);
+}
+EXPORT_SYMBOL(__drm_printfn_debug);
+
 /**
  * drm_printf - print to a &drm_printer stream
  * @p: the &drm_printer