drm/i915/guc: Don't enable scheduling on a banned context, guc_id invalid, not registered

When unblocking a context, do not enable scheduling if the context is
banned, guc_id invalid, or not registered.

v2:
 (Daniele)
  - Add helper for unblock

Fixes: 62eaf0ae ("drm/i915/guc: Support request cancellation")
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: John Harrison <John.C.Harrison@Intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210909164744.31249-10-matthew.brost@intel.com