drm/i915/guc: skip scrub_ctbs selftest if reset is disabled

The test needs GT reset to trigger the scrubbing logic, so we can only
run it when reset is enabled.

Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: John Harrison <john.c.harrison@intel.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: John Harrison <John.C.Harrison@Intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220708224158.929327-1-daniele.ceraolospurio@intel.com