drm/sde: add support for enabling scrambling feature

In order to support 4K@60Hz through HDMI display, driver needs to
enable scrambling feature from HDMI controller and communicate
it with sink device through DDC.

Change-Id: I0b08b6462d19044c4cffed10b0429e7f5d7285f6
Signed-off-by: Jin Li <jinl@codeaurora.org>