disp: msm: sde: fix null access for wb modes

Writeback modes are passed from DRM driver client and it may provide
invalid configuration. Add null checks for sde_wb_config provided by
client to avoid null access in subsequent calls.

Change-Id: I0924f8907d98e2ecb891cfc0c09191823d9033e8
Signed-off-by: Lakshmi Narayana Kalavala <lkalaval@codeaurora.org>