drm/i915: Move final cleanup of drm_i915_private to i915_driver_destroy

Introduce a complementary function to i915_driver_create() to undo all
that is created.

Suggested-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
Reviewed-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180905140921.17467-2-chris@chris-wilson.co.uk