msm: mdss: add support to read hpd state from sysfs node

This change adds support to read HPD state from sysfs node,
which will be helpful in automated test cases where multiple
HPD operations are performed on HDMI TV.

Change-Id: I39d19447c01d11a54a8e54c661b2dd177310c205
Signed-off-by: Sandeep Panda <spanda@codeaurora.org>