msm: mdss: Sanitize input to writeback kickoff

The input frame buffer structure needs to be sanity checked for
the presence of ctl. It avoids initiating kickoff sequence
with an unattached ctl which can lead to a kernel panic.

CRs-fixed: 566720
Change-Id: I4b5cb7983092853aa2286bddee051a5e250e3744
Signed-off-by: Sree Sesha Aravind Vadrevu <svadrevu@codeaurora.org>