drm/i915: make intel_aux_display_runtime_get get runtime PM, not PC8

Because we merged the PC8 and runtime PM features, so calling
intel_runtime_pm_get now has the same meaning, and we plan to just
remove hsw_disable_package_c8 for this exact reason.

My first patch tried to completely kill
intel_aux_display_runtime_get/put, because I was assuming that whoever
needed more than just runtime PM would have to get the appropriate
power domain instead of that, but it seems some people still want the
intel_aux_display_runtime_get abstraction, so keep it until someone
else tries to replace it with the more-standard power domain calls.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>