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. Also, remove an instance where the buffer used value
is incorrectly set as 0.

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