drm/i915: Introduce separate status variable for RC6 and LLC ring frequency setup

Defined new struct intel_rc6 to hold RC6 specific state and
intel_ring_pstate to hold ring specific state.

v2: s/intel_ring_pstate/intel_llc_pstate. Removed checks from
autoenable_* functions. (Chris)

Signed-off-by: Sagar Arun Kamble <sagar.a.kamble@intel.com>
Cc: Imre Deak <imre.deak@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Radoslaw Szwichtenberg <radoslaw.szwichtenberg@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/1507360055-19948-13-git-send-email-sagar.a.kamble@intel.com
Acked-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20171010213010.7415-12-chris@chris-wilson.co.uk