drm/msm/dp: add hook_plugged_cb hdmi-codec op for MSM DP driver

Add the hook_plugged_cb op for the MSM DP driver to signal connect
and disconnect events to the hdmi-codec driver which in-turn shall
notify the audio subsystem to start a new or teardown an existing
session.

Changes in v2: none
Changes in v3: none
Changes in v4: rebase on top of latest patchset of dependency
Changes in v5: rebase on top of latest patchset of dependency
Changes in v6: none

Signed-off-by: Abhinav Kumar <abhinavk@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@chromium.org>