drm/i915: Refactor marking a request as EIO

When wedging the device, we cancel all outstanding requests and mark
them as EIO. Rather than duplicate the small function to do so between
each submission backend, export one.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Andi Shyti <andi.shyti@intel.com>
Reviewed-by: Andi Shyti <andi.shyti@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210109163455.28466-3-chris@chris-wilson.co.uk