msm: synx: fix merged synx sharing across processes

Import of merged synx obj will fail if the release on merged
synx obj by the exporting process finishes before import.
Even though the backed dma fence is reference counted, synx
metadata for the merged obj is not. So release would remove
synx data resulting in failure of import function.
This change saves information of exported synx handles in
a list (this would work for both merged and individual synxs)
which wont be cleared until successfully imported.

Change-Id: I18007112d469db55667b7604423d78a2febc706a
Signed-off-by: Sumukh Hallymysore Ravindra <shallymy@codeaurora.org>