drm/i915/selftests: ring all doorbells in igt_guc_doorbells

We currently verify that all doorbells can be registered with GuC and
HW but don't check that all works as expected after a db ring.

Do a nop ring of all doorbells to make sure we haven't misprogrammed
any WQ or stage descriptor data. This will also help validating
upcoming changes in the db programming flow.

Cc: Michel Thierry <michel.thierry@intel.com>
Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Reviewed-by: Michel Thierry <michel.thierry@intel.com>
Acked-by: Katarzyna Dec <katarzyna.dec@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20180827223614.22789-1-daniele.ceraolospurio@intel.com