disp: msm: dp: deregister altmode notifier

Deregister the altmode notifier in the client teardown path since
the client will no longer be available to handle altmode
registration.

Change-Id: I06c6c61d4eb0b0fc9e9c1df1da4a0c86e4556de1
Signed-off-by: Tatenda Chipeperekwa <tatendac@codeaurora.org>