drm/xe/xe_gt_idle: use GT forcewake domain assertion

The rc6 registers used in disable_c6 function belong
to the GT forcewake domain. Hence change the forcewake
assertion to check GT forcewake domain.

v2: add fixes tag (Himal)

Fixes: 975e4a37

 ("drm/xe: Manually setup C6 when skip_guc_pc is set")
Signed-off-by: Riana Tauro <riana.tauro@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Himal Prasad Ghimiray <himal.prasad.ghimiray@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240606100842.956072-2-riana.tauro@intel.com
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>