msm: mdss: add sysfs node to disable panel

For power measurements, it is needed to disable
the panel in order to determine the delta
between the consumption with and without
the panel and it's dependencies.
This change adds a sysfs note to disable
the panel and dependencies without disturbing
any of the dsi/mdp configurations.

CRs-Fixed: 878591
Change-Id: I57c1f49aa57f645cb96714edd2f297fa06187c03
Signed-off-by: Ingrid Gallardo <ingridg@codeaurora.org>