drm/i915/guc: Add read/write helpers for ADS blob

Add helpers on top of iosys_map_read_field() /
iosys_map_write_field() functions so they always use the right
arguments and make code easier to read.

Cc: Matt Roper <matthew.d.roper@intel.com>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Cc: Daniel Vetter <daniel@ffwll.ch>
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: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Matt Atwood <matthew.s.atwood@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220216174147.3073235-6-lucas.demarchi@intel.com