MAINTAINERS: Updates for drm/i915

Jani for co-maintainer!

Jani has been a really active bug-scrubber in the past few months.
I've asked him whether he wants to do this in a more official capacity
and he agreed. I've already chatted with Dave and Jesse and they
support this.

Note that everyone can't now just relax because "Jani will do all the
bug scrubbing" - au contraire expect more nagging and poking now that
we have more bandwidth.

Longer-term the plan is to share more of the maintainer duties, but we
need to fix up the infrastructure a bit first (like moving the git
repo to a common location).

While at it also add the newly set-up patchwork instance.

Cc: Dave Airlie <airlied@gmail.com>
Cc: Jesse Barnes <jbarnes@virtuousgeek.org>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>