drm/i915/uc: Move fw path check to fetch_uc_fw()

There is no reason to separately check for valid fw path before
we try to fetch it. Let the fetch function take care of this.

Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Arkadiusz Hiler <arkadiusz.hiler@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Arkadiusz Hiler <arkadiusz.hiler@intel.com>
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170330112115.120240-2-michal.wajdeczko@intel.com