drm/i915: Use drm_i915_private directly from debugfs

The void *data passed to debugfs callbacks is actually the
drm_i915_private pointer, so use it thusly and avoid the to_i915(dev)
indirection.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/20170407194220.821-1-chris@chris-wilson.co.uk


Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>