mlx5_core: Re-add MLX5_DEV_CAP_FLAG_ON_DMND_PG flag

In commit 0c7aac854f52 ("net/mlx5_core: Remove unused dev cap enum
fields"), the flag MLX5_DEV_CAP_FLAG_ON_DMND_PG was removed.
Unfortunately the on-demand paging changes actually use it, so re-add
the missing flag.

Signed-off-by: Roland Dreier <roland@purestorage.com>