drm/i915: Error state should print /sys/kernel/debug

/sys/kernel/debug has more or less been the standard location of debugfs
for several years now. Other parts of DRM already use this location, so
we should as well.

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Reviewed-by: Carl Worth <cworth@cworth.org>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
[danvet: split up long line.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>