drm/i915: Make the hexdump row offset visually distinct

Currently we use %08x for the row offset, and %08x for the binary
contents of the buffer. This makes it very easily to confuse the two, so
switch to using [%04x] for the start-of-row offset.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180614094103.18025-3-chris@chris-wilson.co.uk