gpu: drm: nouveau: Use list_{next/prev}_entry instead of list_entry

It's better to use list_entry instead of list_{next/prev}_entry
as it makes the code more clear to read.
This patch replace list_entry with list_{next/prev}_entry.

Signed-off-by: Arushi Singhal <arushisinghal19971997@gmail.com>
Acked-by: Ben Skeggs <bskeggs@redhat.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/1522000893-5331-3-git-send-email-arushisinghal19971997@gmail.com