drm/xe/hdcp: Check GSC structure validity

Sometimes xe_gsc is not initialized when checked at HDCP capability
check. Add gsc structure check to avoid null pointer error.

Signed-off-by: Suraj Kandpal <suraj.kandpal@intel.com>
Reviewed-by: Dnyaneshwar Bhadane <dnyaneshwar.bhadane@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240722064451.3610512-4-suraj.kandpal@intel.com