diag: Do not add duplicate buffers to list in MEMORY_DEVICE_MODE

Diag maintains a buffer table in MEMORY_DEVICE_MODE for draining data
to userspace. Ensure that there are no duplicates while adding buffers
to this table.

Change-Id: Idcfaed210ebb085b5086883375a2655dede10ba2
Signed-off-by: Ravi Aravamudhan <aravamud@codeaurora.org>