msm: mdss: move ULPS configuration to DSI clock control

DSI Ultra Low Power State (ULPS) mode is property of the DSI data
and clock lanes. Current implementation requires the MDP command
interface to send explicit events to the DSI controller to config
ULPS mode. However, it would be a lot more efficient if the DSI
controller would independently decide to enter/exit ULPS based on
whether the link clocks are enabled or not. This also ensures that
if any DCS commands need to be sent when ULPS is enabled, the DSI
controllers will exit ULPS accordingly.

Change-Id: If8d5b7039b5e104bcee5304c7c0ddb3cdd5bbcbc
Signed-off-by: Aravind Venkateswaran <aravindh@codeaurora.org>