msm: sde: Fix variable initialization in SDE rotator

Properly initialize local variable in the SDE rotator WB done handler.
Under error condition, it is possible that the handler access the
uninitialized variable.

CRs-Fixed: 1110015
Change-Id: I4c76fd6400c528f5dd7773aa9b3718dd86b2b01a
Signed-off-by: Benjamin Chan <bkchan@codeaurora.org>