msm: mdss: reset ping-pong registers when display is turned off

Ping-pong registers are used for tear check when command mode panel
is used. They are used for frame buffer compression configuration if
panel supports it. But if video mode panel is used, like HDMI or eDP,
those registers should be at their reset value otherwise it can cause
unpredictable hw state and results in under-runs. To avoid this, reset
ping-pong registers when panel is turned off.

Change-Id: If792c71ac584550a24fd3424481b34179fbf1d77
Signed-off-by: Ujwal Patel <ujwalp@codeaurora.org>