drm/vc4: Fix wrong printk format in vc4_bo_stats_debugfs()

vc4->purgeable.size and vc4->purgeable.purged_size are size_t fields
and should be printed with a %zd specifier.

Fixes: b9f19259 ("drm/vc4: Add the DRM_IOCTL_VC4_GEM_MADVISE ioctl")
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Reviewed-by: Gustavo Padovan <gustavo.padovan@collabora.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20171101095731.14878-1-boris.brezillon@free-electrons.com