drm/i915: Never return 0 if not all requests retired

Users of intel_gt_retire_requests_timeout() expect 0 return value on
success.  However, we have no protection from passing back 0 potentially
returned by a call to dma_fence_wait_timeout() when it succedes right
after its timeout has expired.

Replace 0 with -ETIME before potentially using the timeout value as return
code, so -ETIME is returned if there are still some requests not retired
after timeout, 0 otherwise.

v3: Use conditional expression, more compact but also better reflecting
    intention standing behind the change.

v2: Move the added lines down so flush_submission() is not affected.

Fixes: f33a8a51

 ("drm/i915: Merge wait_for_timelines with retire_request")
Signed-off-by: Janusz Krzysztofik <janusz.krzysztofik@linux.intel.com>
Reviewed-by: Andrzej Hajda <andrzej.hajda@intel.com>
Cc: stable@vger.kernel.org # v5.5+
Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221121145655.75141-3-janusz.krzysztofik@linux.intel.com
(cherry picked from commit f301a29f

)
Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>