drm/gem: Add ww_acquire_ctx support to drm_gem_lru_scan()

If the callback is going to have to attempt to grab more locks, it is
useful to have an ww_acquire_ctx to avoid locking order problems.

Why not use the drm_exec helper instead?  Mainly because (a) where
ww_acquire_init() is called is awkward, and (b) we don't really
need to retry after backoff, we can just move on to the next object.

Signed-off-by: Rob Clark <robdclark@chromium.org>
Signed-off-by: Rob Clark <robin.clark@oss.qualcomm.com>
Tested-by: Antonino Maniscalco <antomani103@gmail.com>
Reviewed-by: Antonino Maniscalco <antomani103@gmail.com>
Patchwork: https://patchwork.freedesktop.org/patch/661463/