msm: mdss: Avoid null pointer deref in writeback terminate

Writeback terminate can be called without the allocation of ctl
especially when a file descriptor needs to be closed for ioctl
failures.

CRs-fixed: 489296
Change-Id: I907d3f9ba1825829b3198f66c918071e59d37968
Signed-off-by: Sree Sesha Aravind Vadrevu <svadrevu@codeaurora.org>