The new GuC interface introduces an MMIO H2G command,
INTEL_GUC_ACTION_RESET_CLIENT, which is used to implement suspend. This
MMIO tears down any active contexts generating a context reset G2H CTB
for each. Once that step completes the GuC tears down the CTB
channels. It is safe to suspend once this MMIO H2G command completes
and all G2H CTBs have been processed. In practice the i915 will likely
never receive a G2H as suspend should only be called after the GPU is
idle.

Resume is implemented in the same manner as before - simply reload the
GuC firmware and reinitialize everything (e.g. CTB channels, contexts,
etc..).

v2:
 (Michel / John H)
  - INTEL_GUC_ACTION_RESET_CLIENT 0x5B01 -> 0x5507

Cc: John Harrison <john.c.harrison@intel.com>
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Reviewed-by: John Harrison <John.C.Harrison@Intel.com>
Signed-off-by: John Harrison <John.C.Harrison@Intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210727002348.97202-12-matthew.brost@intel.com