drm/i915/huc: Load GSC-enabled HuC via DMA xfer if the fuse says so

In the previous patch we extracted the offset of the legacy-style HuC
binary located within the GSC-enabled blob, so now we can use that to
load the HuC via DMA if the fuse is set that way.
Note that we now need to differentiate between "GSC-enabled binary" and
"loaded by GSC", so the former case has been renamed to "has GSC headers"
for clarity, while the latter is now based on the fuse instead of the
binary format. This way, all the legacy load paths are automatically
taken (including the auth by GuC) without having to implement further
code changes.

v2: s/is_meu_binary/has_gsc_headers/, clearer logs (John)

v3: split check for GSC access, better comments (John)

Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Alan Previn <alan.previn.teres.alexis@intel.com>
Cc: John Harrison <John.C.Harrison@Intel.com>
Reviewed-by: John Harrison <John.C.Harrison@Intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230531235415.1467475-4-daniele.ceraolospurio@intel.com