msm: mdss: reuse pipes from cleanup pool

By reusing pipes from cleanup pool, there is no need to
block user client while commit with cleanup is in progress.
This allows async calls from user space during prepare
cycle.

Change-Id: I688196320ed5ffad36b9f09b89fd512035887d6d
Signed-off-by: Ingrid Gallardo <ingridg@codeaurora.org>