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