drm/i915/uc: Simplify firmware path handling

drm/i915/uc: Simplify firmware path handling

Currently fw->path values can represent one of three possible states:

 1) NULL - device without the uC
 2) '\0' - device with the uC but have no firmware
 3) else - device with the uC and we have firmware

Second case is used only to WARN at a later stage.

We can WARN right away and merge cases 1 and 2.

Code can be even further simplified and common (HuC/GuC logic) happening
right before the fetch can be offloaded to the common function.

v2: fewer temporary variables, more straightforward flow (M. Wajdeczko)
v3: DRM_ERROR instead of WARN (M. Wajdeczko)
v4: coding standard (J. Lahtinen)
v5: non-trivial rebase
v6: remove path check, we are checking fetch status (M. Wajdeczko)

Cc: Anusha Srivatsa <anusha.srivatsa@intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Michal Winiarski <michal.winiarski@intel.com>
Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Signed-off-by: Arkadiusz Hiler <arkadiusz.hiler@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>