drm/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: Idfa3f8e5207baefb61b7ab39e684c4b92376d7b3
Signed-off-by: Raviteja Tamatam <travitej@codeaurora.org>