plankton: treat USB mode as DP detached
plankton: treat USB mode as DP detached

When a DP cable is attached, if we toggle the USB/DP mux to USB mode,
the DP data link is broken. In this case, we should send a HPD event to
notify the host. Similarily, when we switch back to DP mode when a cable
is attached, we should send a HPD attachment event and then a HPD IRQ to
notify the host that the DP link is ready.

BRANCH=None
BUG=chrome-os-partner:34296
TEST=Connect Plankton to Samus. Perform the following tests:
      1. Attach a cable, and switch between USB and DP mode.
      2. Attach a cable in DP mode, switch to USB mode, unplug the
         cable, and then switch back to DP mode.
      3. Similar to 2 but start without the cable.

Change-Id: I5db0c036ce45850b2f4ca142957f4a235673df6f
Signed-off-by: Vic Yang <5fd92abeee651458f98e2856f4f200ee971cee4d@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/237604
Reviewed-by: Alec Berg <1a4429eeda29095a12e19a73405f80f5b7f37a63@chromium.org>
Commit-Queue: Todd Broch <035f5b1f817cbb94019cec357b99778115602b63@chromium.org>
Tested-by: Todd Broch <035f5b1f817cbb94019cec357b99778115602b63@chromium.org>

fourier49/BIZ_EC,coreboot/chrome-ec,akappy7/ChromeOS_EC_LED_Diagnostics,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,akappy7/ChromeOS_EC_LED_Diagnostics,fourier49/BZ_DEV_EC,akappy7/ChromeOS_EC_LED_Diagnostics,coreboot/chrome-ec,coreboot/chrome-ec,fourier49/BZ_DEV_EC,fourier49/BZ_DEV_EC,md5555/ec,akappy7/ChromeOS_EC_LED_Diagnostics,md5555/ec,fourier49/BZ_DEV_EC,fourier49/BIZ_EC,fourier49/BIZ_EC,md5555/ec,akappy7/ChromeOS_EC_LED_Diagnostics,md5555/ec,fourier49/BIZ_EC