The ADS initialitazion was using 2 passes to calculate the regset sent
to GuC to initialize each engine: the first pass to just have the final
object size and the second to set each register in place in the final
gem object.

However in order to maintain an ordered set of registers to pass to guc,
each register needs to be added and moved in the final array. The second
phase may actually happen in IO memory rather than system memory and
accessing IO memory by simply dereferencing the pointer doesn't work on
all architectures. Other places of the ADS initializaition were
converted to use the iosys_map API, but here there may be a lot more
accesses to IO memory. So, instead of following that same approach,
convert the regset initialization to calculate the final array in 1
pass and in the second pass that array is just copied to its final
location, updating the pointers for each engine written to the ADS blob.

One important thing is that struct temp_regset now have
different semantics: `registers` continues to track the registers of a
single engine, however the other fields are updated together, according
to the newly added `storage`, which tracks the memory allocated for
all the registers. So rename some of these fields and add a
__mmio_reg_add(): this function (possibly) allocates memory and operates
on the storage pointer while guc_mmio_reg_add() continues to manage the
registers pointer.

On a Tiger Lake system using enable_guc=3, the following log message is
now seen:

	[  187.334310] i915 0000:00:02.0: [drm:intel_guc_ads_create [i915]] Used 4 KB for temporary ADS regset

This change has also been tested on an ARM64 host with DG2 and other
discrete graphics cards.

v2 (Daniele):
  - Fix leaking tempset on error path
  - Add comments on struct temp_regset to document the meaning of each
    field

Cc: Matt Roper <matthew.d.roper@intel.com>
Cc: John Harrison <John.C.Harrison@Intel.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Signed-off-by: NLucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: NDaniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220208070141.2095177-3-lucas.demarchi@intel.com