drm/i915: Reduce test_and_set_bit to set_bit in i915_request_submit()

Avoid the full blown memory barrier of test_and_set_bit() by noting the
completed request and removing it from the lists.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Andi Shyti <andi.shyti@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210114135612.13210-5-chris@chris-wilson.co.uk