ARM: dts: msm: enable dsi phy idle mode for sdm845

Enable DSI phy idle mode for sdm845. This allows DSI
to disable phy regulator during idle or if data rate is
below per lane threshold defined in phy device node.

Change-Id: I8eda9c63e99955fd5b2c4588e9dc3255065876b9
Signed-off-by: Alan Kwong <akwong@codeaurora.org>