disp: msm: dp: send hpd notification before updating mst_active

When processing mst hpd low, the driver is clearing mst_active before
triggering hpd notification. The hpd notifier is common for both
sst and mst and since mst_active is cleared it incorrectly treats
this as sst unplug.

This change switches the order of these operations to trigger hpd
notification before clearing mst_active.

Change-Id: I28f90da699e4f2fe177a4e4cfd1d9f03957c3176
Signed-off-by: Rajkumar Subbiah <rsubbia@codeaurora.org>