drm/i915: don't get/put PC8 when getting/putting power wells

Because we already get/put runtime PM every time we get/put any power
domain, and now PC8 and runtime PM are the same thing.

With this, we can also now kill the hsw_{en,dis}able_package_c8
functions.

v2: - Rebase.
v3: - Rebase.
v4: - 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>