drm/i915/gsc: fixes and updates for GSC memory allocation

A few fixes/updates are required around the GSC memory allocation and it
is easier to do them all at the same time. The changes are as follows:

1 - Switch the memory allocation to stolen memory. We need to avoid
accesses from GSC FW to normal memory after the suspend function has
completed and to do so we can either switch to using stolen or make sure
the GSC is gone to sleep before the end of the suspend function. Given
that the GSC waits for a bit before going idle even if there are no
pending operations, it is easier and quicker to just use stolen memory.

2 - Reduce the GSC allocation size to 4MBs, which is the POR requirement.
The 8MBs were needed only for early FW and I had misunderstood that as
being the expected POR size when I sent the original patch.

3 - Perma-map the GSC allocation. This isn't required immediately, but it
will be needed later to be able to quickly extract the GSC logs, which are
inside the allocation. Since the mapping code needs to be rewritten due to
switching to stolen, it makes sense to do the switch immediately to avoid
having to change it again later.

Note that the explicit setting of CACHE_NONE for Wa_22016122933 has been
dropped because that's the default setting for stolen memory on !LLC
platforms.

v2: only memset the memory we're not overwriting (Alan)

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>
Cc: Vinay Belgaumkar <vinay.belgaumkar@intel.com>
Reviewed-by: Alan Previn <alan.previn.teres.alexis@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230612181529.2222451-2-daniele.ceraolospurio@intel.com