msm: sde: release LK's pool when LK exits

LK's pool is reserved for running on its code stack. So once
LK exits, the memory region it stays on should be released
and reused by system.

Change-Id: Iec5af69573eb633de48c931a052bd596f5ed3bda
Signed-off-by: Guchun Chen <guchunc@codeaurora.org>