drm/i915: initialize dpio_lock spin lock

This thing is killing lockdep.

Signed-off-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
[Jani: move the init next to the other spin lock inits]
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Acked-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>