drm/i915: Remove i915_debugfs_unregister()

drm_debugfs_cleanup() now removes all minor->debugfs_list entries
automatically, so no need to do this explicitly. Additionally it
uses debugfs_remove_recursive() to clean up the debugfs files,
so no need for adding fake drm_info_node entries.

Cc: daniel.vetter@intel.com
Cc: jani.nikula@linux.intel.com
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20170126225621.12314-20-noralf@tronnes.org