dax: add a user priority mechanism

When multiple DAX modules are running simultaneously, memory resources
may be insufficient. In this case, memory requests from the DAX module
with the highest priority will be fulfilled first. For DAX modules that
cannot obtain memory resources in a timely manner, `dax_process` will
operate in passthrough mode.

Signed-off-by: Jun Lai <jun.lai@dolby.com>

Author: checkupup