msm: mdss: hdmi: Support for HPD enable/disable

Provide support for HPD (Hot Plug Detection) enable/disable
using sysfs node. Write 3 to sysfs node "hpd" to disable the
HPD until written 4 to enable it back.

Change-Id: Ib80ea1f55b859e7935e7f187dc06222ab6bea2d8
Signed-off-by: Ajay Singh Parmar <aparmar@codeaurora.org>