drm/i915: rename __hsw_do_{en, dis}able_pc8

After we removed all the intermediate abstractions, we can rename
these functions to just hsw_{en,dis}able_pc8.

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>