msm: ocmem: Fix a memory leak when destroying regions.

Slab memory allocated using idr_pre_get() was not being
freed as the call to destroy the idr was missing.

Change-Id: Ied0f3d454db845b74b606d264368dd8785e8c5d0
Signed-off-by: Naveen Ramaraj <nramaraj@codeaurora.org>