drm/i915/gt: Suppress the error message for GT init failure on error injection

If we injected an error (such as pretending the GuC firmware was
broken), then suppress the error message as it is expected and our CI
complains if it sees any *ERROR*.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200603104657.25651-1-chris@chris-wilson.co.uk