drm/i915: Rename intel_enable_rc6 to intel_rc6_enabled

drm/i915: Rename intel_enable_rc6 to intel_rc6_enabled

This function gives the status of RC6, whether disabled or if
enabled then which state. intel_enable_rc6 will be used for
enabling RC6 in the next patch.

v2: Rebase.

v3: Rebase.

Signed-off-by: Sagar Arun Kamble <sagar.a.kamble@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Imre Deak <imre.deak@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Radoslaw Szwichtenberg <radoslaw.szwichtenberg@intel.com> #1
Reviewed-by: Ewelina Musial <ewelina.musial@intel.com> #1
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/1507360055-19948-10-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/20171010213010.7415-9-chris@chris-wilson.co.uk