drm/i915/huc: Prepare for GSC-loaded HuC

HuC loading via GSC is performed via a PXP command sent through the mei
modules, so we need both MEI_GSC and MEI_PXP to be available. Given that
the GSC will do both the transfer and the authentication, the legacy HuC
loading paths can be safely skipped.
Also note that the GSC-loaded HuC survives GT reset.

v2: move the huc_is_authenticated() function to this patch.

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/20220504204816.2082588-4-daniele.ceraolospurio@intel.com