drm/i915: ensure lru ordering of fence_list

drm/i915: ensure lru ordering of fence_list

The fence_list should be lru ordered for otherwise we might try
to steal a fence reg from an active object even though there are
fences from inactive objects available. lru ordering was obeyed
for gpu access everywhere save when moving dirty objects from
flushing_list to active_list.

Fixing this cause the code to indent way to much, so I've extracted
the flushing_list processing logic into its on function.

Signed-off-by: Daniel Vetter <[email protected]>
Reviewed-by: Chris Wilson <[email protected]>
Signed-off-by: Eric Anholt <[email protected]>