drm/i915/huc: drop intel_huc_is_authenticated

The function name is confusing, because it doesn't check the actual auth
status in HW but the SW status. Given that there is only one user (the
huc_auth function itself), just get rid of it and use the FW status
checker directly.

Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Reviewed-by: Alan Previn <alan.previn.teres.alexis@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220504204816.2082588-2-daniele.ceraolospurio@intel.com