drm/i915: rename dev_priv info to __info to avoid usage

Encourage use of INTEL_INFO() to access dev_priv->info to not accumulate
more direct users of ->info, making further changes easier.

Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/5f5d81880046331f77624d00278528abc1cf30c6.1546267488.git.jani.nikula@intel.com