drm/vc4: Use drm_printer for the debugfs and runtime bo stats output.

Now I can extend the stats without more copy and pasting between the
two.

Signed-off-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20190220210343.28157-4-eric@anholt.net
Reviewed-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com>