drm/i915: Drop error handling from dma_fence_work

Asynchronous command parsing was the only thing which ever returned a
non-zero error.  With that gone, we can drop the error handling from
dma_fence_work.

Signed-off-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Jon Bloomfield <jon.bloomfield@intel.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210714193419.1459723-5-jason@jlekstrand.net