drm/i915: Move drm_add_fake_info_node() higher in the file

Following commit needs drm_add_fake_info_node() higher in the file to
avoid having a forward declaration. Move this helper near the top of the
file.

This also makes the next commit diff a bit easier to review.

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>