msm: mdss: Add support for ULPS mode for DSI

For smart panels that can refresh display from their internal
RAMs, it is possible to configure the DSI clock and data lanes
in Ultra Low Power State (ULPS) during idle static screen
usecase. Add support for this feature.

Change-Id: I4e94d6a0201262f0675322efc9e39dd93c86edda
Signed-off-by: Aravind Venkateswaran <aravindh@codeaurora.org>