msm_fb: hdmi: CEC Message queue flush

CEC message queue should be flushed when CEC
function is enabled. If there are residue CEC
message, they might cause collision at message
queue.

CRs-fixed: 338811
Change-Id: I6268528417d660fab48818ae67e94e169abbb217
Signed-off-by: Ajay Singh Parmar <aparmar@codeaurora.org>