drm/i915/uc: correctly track uc_fw init failure

The FAILURE state of uc_fw currently implies that the fw is loadable
(i.e init completed), so we can't use it for init failures and instead
need a dedicated error code.

Note that this currently does not cause any issues because if we fail to
init any of the firmwares we abort the load, but better be accurate
anyway in case things change in the future.

Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211211000756.1698923-2-daniele.ceraolospurio@intel.com