ARM: dts: msm: add proxy vote for DSI supplies for sm6150

For continuous splash screen feature, the display needs
to be kept alive during the kernel boot up. Add proxy
vote for all the regulator supplies needed to keep
the display alive.

Change-Id: I8ea2dd5beae799b580c0c17f5106835b5c9f13be
Signed-off-by: Sandeep Panda <spanda@codeaurora.org>