drm/msm/dsi-staging: add property to set DSI channel for DCS command

Some panel only support DSI1 to send DCS command, so we need to choose
the right DSI channel to send DCS read command for ESD checks.

Change-Id: I634013e28554acc56f6b8094ccdf1337d23c8679
Signed-off-by: Yuan Zhao <yzhao@codeaurora.org>
Signed-off-by: Sandeep Panda <spanda@codeaurora.org>