drm/msm/sde: decouple sde_fence_context from sde_connector

Avoid the possibility of accessing invalid sde_fence_context
object after sde_connector object is released. Decouple the
sde_fence_context object from sde_connector and allocate
run-time memory for fence object and deallocate once all the
fence operations are complete and kref for fence is zero.

Change-Id: I4f8a4d6f5df3443ecf4578abaedb6697f375dc11
Signed-off-by: Govinda Rajulu Chenna <gchenna@codeaurora.org>