drm/i915: Create generic functions to control RC6, RPS

Prepared generic functions intel_enable_rc6, intel_disable_rc6,
intel_enable_rps and intel_disable_rps functions to setup RC6/RPS
based on platforms.

v2: Make intel_enable/disable_rc6/rps static. (Chris)

v3: Added lockdep_assert_held(dev_priv->pcu_lock) in new generic
functions. (Chris)
Removed WARN_ON(&dev_priv->pcu_lock) from lower level functions as generic
function now has lockdep_assert. Rebase.

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/1507360055-19948-12-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/20171010213010.7415-11-chris@chris-wilson.co.uk