fastrpc: rpcmem: fix double refcounting dmabuf

BUG=b:138978210
TEST=label_image -a 1 in batch mode

Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Change-Id: I68c42288b3747995eda556a5a6215cb0419c06f4
Signed-off-by: Tharun Kumar Merugu <mtharu@codeaurora.org>
Signed-off-by: Abhinav Asati <asatiabhi@codeaurora.org>
Signed-off-by: Mohammed Nayeem Ur Rahman <mohara@codeaurora.org>