msm: mdss: dp: fix handling of device shutdown with cable connected

When the source is powered off with an external sink connected, it
will result in a call to blank and power off the source. In the
current implementation, sink is not transitioned to D3 power mode
prior to turning off the source. This can result in the sink
reporting signal lock failures as the source if powered off,
resulting in an HPD IRQ interrupt. If this interrupt is handled
after the DP controller is powered off, it can lead to unclocked
register accesses. Fix this by ensuring that sink is powered off
prior to powering off the source. In addition, ensure that no more
attention events are handled once the DP controller is powered off.

Change-Id: I08558229f7c3e603904527de58a39039b3d7615e
Signed-off-by: Aravind Venkateswaran <aravindh@codeaurora.org>
Signed-off-by: Padmanabhan Komanduru <pkomandu@codeaurora.org>