drm/msm/dp: add extra 100ms sleep when MSTM_CTRL is not cleared

Add extra sleep between SST and MST mode if MSTM_CTRL is not cleared
at hpd to make sure dongle exit from previous MST state.

CRs-Fixed: 2411911
Change-Id: I5b1785c71a91df95167bf5083e9d752a275872a8
Signed-off-by: Xiaowen Wu <wxiaowen@codeaurora.org>
Signed-off-by: Fuad Hossain <fhossain@codeaurora.org>