disp: msm: dp: skip powering down dp host in mst mode

If DP is running in mst mode, DP host should not power down during
unprepare call as aux need keep alive to handle further hot-plug
sideband message. Unless it is a PM suspend scenario where it is
required to deinit the host.

Change-Id: I208894e1cbc75e727044c196194b0a4e3936921e
Signed-off-by: Xiaowen Wu <wxiaowen@codeaurora.org>
Signed-off-by: Sankeerth Billakanti <sbillaka@codeaurora.org>