drm/i915/uc: Move intel_uc_fw_fetch() to intel_uc.c

The file fits better.

Additionally rename it to intel_uc_prepare_fw(), as the function does
more than simple fetch.

`obj` cleanup in the function is also fixed (i.e. removed). In the fail
scenario it was always 'put' but there's no possible flow that
initializes the obj properly and then goes to the fail label.

v2: remove second declaration, reorder (M. Wajdeczko)
v3: non-trivial rebase
v4: remove obj cleanup in the fail scenario (C. Wilson)

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
Signed-off-by: Arkadiusz Hiler <arkadiusz.hiler@intel.com>
Reviewed-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>