msm:mdss: Add support for show blank sysfs entry

Currently there is no support for show blank sysfs entry
in mdss. Hence, added this entry to check if panel is
power on or off.

Change-Id: Ifa50bf373590556f88b8f8196353dffefe2d2559
Signed-off-by: Jayant Shekhar <jshekhar@codeaurora.org>