drm/i915/guc: Replace WARN with a DRM_ERROR

Replace the WARN with a simple if() + error message to squech the sparse
warning that entire wait_for() macro was being stringified:

drivers/gpu/drm/i915/intel_guc_submission.c:658:9: error: too long token expansion

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190408091728.20207-2-chris@chris-wilson.co.uk