msm: synx: fix synx metadata cleanup

An un-released merged synx obj metadata is not cleared
currently, resulting in resource exhaustion. Also, if
more than one reference is obtained to an individual
synx obj, and all the references are not released by
the client appropriately, during cleanup, only one
reference is released, which will not clear the metadata
or the allocated memory for the dma fences due to other
remaining references.
Change fixes the metadata cleanup for merged synx objs and
synx obs un-released with multiple references by iterating
till all the references are released and metadata cleared
for each. Also removes any un-imported synx obj data in
import list.

Change-Id: I51c20977ccd1694b4ceb3217ae7d4e97c72bcc7f
Signed-off-by: Sumukh Hallymysore Ravindra <shallymy@codeaurora.org>