drm/i915: enable Package C8+ by default

This should be working, so enable it by default. Also easy to revert.

v2: Rebase, s/allow/enable/.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>