drm/i915/guc: Remove client->submissions

The engine->guc_id is GuC FW defined and it is not guaranteed to be
below I915_NUM_ENGINES, so we shouldn't use it with the i915-defined
client->submissions, as we might overflow.
Instead of fixing it, just get rid of client->submissions, because the
information we get from it is not interesting anymore now that we only
have 1 client.

Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190814002145.29056-1-daniele.ceraolospurio@intel.com