drm/i915/guc: fix GuC suspend/resume

The ENTER/EXIT_S_STATE actions queue the save/restore operation in GuC
FW and then return, so waiting on the H2G is not enough to guarantee
GuC is done.
When all the processing is done, GuC writes 0 to scratch register 14,
so we can poll on that. Note that GuC does not ensure that the value
in the register is different from 0 while the action is in progress
so we need to take care of that ourselves as well.

v2: improve comment, return early on GuC error and improve error
    message (Michal)

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Acked-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20181016224648.2326-1-daniele.ceraolospurio@intel.com