drm/i915/guc: Provide mmio list to be saved/restored on engine reset

The driver must provide GuC with a list of mmio registers
that should be saved/restored during a GuC-based engine reset.
Unfortunately, the list must be dynamically allocated as its size is
variable. That means the driver must generate the list twice - once to
work out the size and a second time to actually save it.

v2:
 (Alan / CI)
  - GEN7_GT_MODE -> GEN6_GT_MODE to fix WA selftest failure

Signed-off-by: John Harrison <John.C.Harrison@Intel.com>
Signed-off-by: Fernando Pacheco <fernando.pacheco@intel.com>
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Signed-off-by: John Harrison <John.C.Harrison@Intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210727002348.97202-16-matthew.brost@intel.com