msm: ipa: Decrease refcount of the pages held in cache before freeing

While freeing the buffers in the transfer ring, the ref_count of the
pages that were allocated from the sys->repl->cache have to be
decremented explicitly to ensure there is no memory leak.

Change-Id: Ie853bf15f084f503e8818d46c72671dbb9ddad7b
Acked-by: Priyadarshini Rajagopal <prajagop@qti.qualcomm.com>
Signed-off-by: Bojun Pan <bojunp@codeaurora.org>