msm: mdss: hdmi: correctly validate the cec msg frame size

HDMI CEC message is being validated against the operand
size (MAX_OPERAND_SIZE). Instead validate against the
frame size (MAX_CEC_FRAME_SIZE).

Change-Id: I9fd66781feedbe9bc1090b0e28ef9d814edcca5a
Signed-off-by: Narender Ankam <nankam@codeaurora.org>