msm: clock-mdss-8974: Proper hdmi set rate

Do not call set rate if the rate is zero. Reset the flag which sets
the rate on cable disconnect so that next cable connect can set the
rate again.

Change-Id: Ieb8eed3760c4b285182ae47d732352771d13b020
Signed-off-by: Ajay Singh Parmar <aparmar@codeaurora.org>