drm/i915/guc: Add golden context to GuC ADS

The media watchdog mechanism involves GuC doing a silent reset and
continue of the hung context. This requires the i915 driver provide a
golden context to GuC in the ADS.

v2:
 (Matthew Brost):
  - Fix memory corruption in shmem_read
 (John H)
  - Use locals rather than defines for LR_* + SKIP_SIZE

Signed-off-by: John Harrison <John.C.Harrison@Intel.com>
Signed-off-by: Matthew Brost <matthew.brost@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/20210727002348.97202-24-matthew.brost@intel.com