drm/i915/gt: Use rc6.supported flag from intel_gt for rc6_enable sysfs

Currently if rc6 is supported, it gets enabled and the sysfs files for
rc6_enable_show and rc6_enable_dev_show uses masks to check information
from drm_i915_private.

However rc6_support functions take more variables and conditions into
consideration and thus these masks are not enough for most of the modern
hardware and it is simpley lyting to the user.

Let's fix it by at least use the rc6.supported flag from intel_gt
information.

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/20240110010302.553597-1-jcescami@wasd.net