We already have the routine we need in drv_suspend. Let's move it to lib
and use it in the mlocking tests. We can also make it a bit faster if we
tweak the initial step and initial amount.
(I think it's safe to assume that we should be able to lock 3/4
of RAM, this cuts the probe time on my 32G SKL - from ~530s to ~180s)

v2: Use available mem, amend step, also lock outside of fork,
    early exit if the assumption is wrong (Chris)
    Update the function name in doc (Ewelina)
v3: Total for pin, available for initial lock (Chris)

Signed-off-by: Michał Winiarski <michal.winiarski@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Ewelina Musial <ewelina.musial@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Ewelina Musial <ewelina.musial@intel.com>