ion: system_secure_heap: Change IOC_DRAIN work_struct to delayed_work

The video client signals to ion that it is done using an amount of memory
using the ioctl ION_IOC_DRAIN. However, the video client may have
previously shared this memory with other clients, such that the reference
count on existing ion_buffers is greater than one. Work around this issue
for now by assuming that the ion_buffers will be freed by the other users
within a short duration after the video client makes the ION_IOC_DRAIN
call.

Change-Id: I113ca1ff26e4949b27879ccf67af6d5f04a67b06
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>