mdss: dsi: Power down DSI phy on panel off

Whenever the primary panel on the device is turned
off, we need to power down the DSI phy in addition to
voting off all the regulators used by the DSI controller.
This avoids the current leakage in low power mode.

Change-Id: Ic53a3a23c957290e66b43cac40aaaaa198719e9d
Signed-off-by: Aravind Venkateswaran <aravindh@codeaurora.org>