msm: synx: support for external dma fence

Changes to support synx framework with both internally
created dma fences and as a wrapper to externally
managed dma fences.
Accepts the external dma fence and binds to existing
synx object through the synx_export api. A new api
added to return the dma fence backing the synx object.
Modifications to object creation, management and cleanup
due to the support for external dma fences. Each synx
object holds single reference to the underlying dma fence
and multiple client references to the synx object is
managed through the reference counts maintained per
client handle.
Added mutex lock to handle the critical section with
synx objects. Non interruptible spin locks will be used
when required to update backing fence.

Change-Id: I94a77436e0c2379e3be5227dfb1ce835dc109ea5
Signed-off-by: Sumukh Hallymysore Ravindra <shallymy@codeaurora.org>