drm/i915: get runtime PM references when the GPU is idle/busy

... instead of PC8 references. Now that both are the same thing and we
are killing PC8, just get the runtime PM reference.

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>