MAINTAINERS: Update drm/i915 git repo

Moved to a common location so that Jani also can push to it, to avoid
moving it every time I go on vacation. Please update autobuilders and
everything else pointing at the drm-intel.git repo, the old one won't
be updated any more.

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