msm: mdss: Add power on/off support for HDMI TX driver

Add support for HDMI panel power on and power off. Open and close
of fb device associated with HDMI panel results in HDMI Tx's
power on and power off respectively.

Change-Id: I9becdffefe681ce104d631e798d196e23dae4995
Signed-off-by: Ujwal Patel <ujwalp@codeaurora.org>