drm/i915/gt: Reflect the true and current status of rc6_enable

The sysfs file is named 'enabled', thus users might want to know the
true state of the RC6 instead of only the indication if the RC6
should be enabled.

Let's use rc6.enable directly instead of rc6.supported.

Signed-off-by: Juan Escamilla <jcescami@wasd.net>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240116172922.3460695-1-jcescami@wasd.net