drm/i915: convert debugfs creation/destruction to table

At least for the common cases where we only need special file
operations. The forcewake file is still rather more special.

v2: Fix up the debugfs unregister code.

v3: Actually squash in the right fixup.

Acked-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>