msm: mdss: Add support for DSI always on

Some DSI to HDMI bridges require the DSI signal to be always on.
This change allows to keep the DSI bus in steady on.

Change-Id: I5df07ec74f5a4a8a6444cf2025f663b6b57bce30
Signed-off-by: Ilia Lin <ilialin@codeaurora.org>
Signed-off-by: Tanya Finkel <tfinkel@codeaurora.org>
Signed-off-by: Bharat Pawar <bpawar@codeaurora.org>
Signed-off-by: Nirmal Abraham <nabrah@codeaurora.org>