msm_fb: hdmi: black out pipes on unset

For HDMI as primary case, we need to blackout the pipes
when they are unset so that any residue is not visible
on the DTV during suspend.

Change-Id: I2bf36f1002dd185c26fe6c4291977737daa73ae1
Signed-off-by: Ajay Singh Parmar <aparmar@codeaurora.org>