drm/msm/sde: remove unused writeback function stubs

Remove unused function stubs from writeback hardware driver.  Client
can check if certain feature or capabilty is supported by first checking
operation function pointer before calling the operation.

Change-Id: I21f7c75ab750c76328e77fbb67eda2a8c5d50123
Signed-off-by: Alan Kwong <akwong@codeaurora.org>