hwc: Moving error log to debug log for non-fatal error

In secure RGB usecase, we mark secure RGB layers for MDP composition
as GPU does not support secure RGB composition even if skip layer
is present. In markLayerForCaching function, when we encounter a skip
layer marked for MDP composition whose handle can be null will flag
"Layer handle is NULL" as error. So we can enable this message only
when MDP debugging logs are enabled.

Change-Id: I8be579636bf1de12a032c289e2542614fbbd375c
1 file changed