diag: dci: Reduce dci table memory foot print

Diag driver uses a table of definite size to maintain information about
the DCI clients. This is allocated when the driver comes up. This memory
is relatively large and is allocated irrespective of the number of
dci clients. This patch changes the implementation from a table to a list.
This allocates a table entry for a client only when they register for DCI.

Change-Id: I7a22536a49c3e640e3e6d86c9731df02c8f04b58
Signed-off-by: Ravi Aravamudhan <aravamud@codeaurora.org>