drm/msm: Add debugfs to trigger shrinker

Just for the purposes of testing.  Write to it the # of objects to scan,
read back the # freed.

Signed-off-by: Rob Clark <robdclark@chromium.org>
Link: https://lore.kernel.org/r/20210614150618.729610-1-robdclark@gmail.com


Signed-off-by: Rob Clark <robdclark@chromium.org>