disp: msm: dp: send hotplug notification on state change

Send the hotplug notification to the user space only when there
is a state change e.g. from connect to disconnect, or vice versa.
This ensures that there are no spurious notifications that are
sent to the user space.

CRs-Fixed: 2478822
Change-Id: Ic3fdc95de5f2573a14cb526203d7e4ab58c567ba
Signed-off-by: Tatenda Chipeperekwa <tatendac@codeaurora.org>