drm/i915: Lift i915_request_show()

Extract i915_request_show for reuse in other request chain pretty
printers.

For a bonus point, quietly change the seqno format from %llx to %lld to
match everywhere else.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201119165616.10834-2-chris@chris-wilson.co.uk