drm/i915: Downgrade device register error if injected

Commit 8f460e2c ("drm/i915: Demidlayer driver loading") which
introduced manual device registration also added a message that is
submitted on device registration failure as an error.  If that failure is
triggered by error injection test, that's an expected error, but CI still
reports it as a bug.  Fix it.

Suggested-by: Krzysztof Niemiec <krzysztof.niemiec@intel.com>
Closes: https://gitlab.freedesktop.org/drm/i915/kernel/-/issues/9820


Cc: Chris Wilson <chris.p.wilson@linux.intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Janusz Krzysztofik <janusz.krzysztofik@linux.intel.com>
Reviewed-by: Krzysztof Niemiec <krzysztof.niemiec@intel.com>
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Signed-off-by: Andi Shyti <andi.shyti@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250314205202.809563-6-janusz.krzysztofik@linux.intel.com