drm/i915: kill dev_priv->pc8.gpu_idle

Since the addition of dev_priv->mm.busy, there's no more need for
dev_priv->pc8.gpu_idle, so kill it.

Notice that when you remove gpu_idle, hsw_package_c8_gpu_idle and
hsw_package_c8_gpu_busy become identical to hsw_enable_package_c8 and
hsw_disable_package_c8, so just use them.

Also, when we boot the machine, dev_priv->mm.busy initially considers
the machine as idle. This is opposed to dev_priv->pc8.gpu_idle, which
considered it busy. So dev_priv->pc8.disable_count has to be
initalized to 1 now.

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