drm/msm/dp: cancel attention work on disconnect

When cable is disconnected, cancel the attention work as well.
Attention work handles all the attention events sent by the
sink device. In that process, it can queue other works based
on the event received. Cancelling attention work makes sure
that the works are not queued further with dp controller powered
off.

CRs-Fixed: 2216312
Change-Id: I8780bfc5cf76d43e432b00acfdbd7f68e5733bba
Signed-off-by: Ajay Singh Parmar <aparmar@codeaurora.org>