drm/i915: kill struct i915_package_c8

The only remaining field of the struct was the lock, which was
useless.

v2: - Rebase.

Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>