disp: msm: dp: fix dp-mst stream double disabled issue

There is a corner case when dp-mst stream is disabled twice:

1. stream 1 is enabled
2. cable plugged out, stream 1 is disabled by kernel
3. cable plugged in, enable stream 0 first
4. disable stream 1

Stream 1 is double disabled and will cause hw crash. This change
will add the check and skip stream_off if already disabled.

Change-Id: Icd1e815a1ed14cd4e784924f3db19b98eb06d571
Signed-off-by: Xiaowen Wu <wxiaowen@codeaurora.org>
Signed-off-by: Sankeerth Billakanti <sbillaka@codeaurora.org>