TCMPv2: Handle DP mode exit.
TCMPv2: Handle DP mode exit.

When DisplayPort alt mode is in effect, and the system is
powered down, send a Mode exit message to cleanly
reset the mode of the partner.

BUG=b:156306710
TEST=Shutdown and power up with type-C monitor connected.
BRANCH=none

Signed-off-by: Andrew McRae <44ad1424cfc8411c96d9181798ca41ae55709dfb@google.com>
Change-Id: I656eeeeaeb517f2d5f49281bdd26e91a5fb6fc6d
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/2219792
Tested-by: Andrew McRae <44ad1424cfc8411c96d9181798ca41ae55709dfb@chromium.org>
Commit-Queue: Andrew McRae <44ad1424cfc8411c96d9181798ca41ae55709dfb@chromium.org>
Reviewed-by: Diana Z <cb2016afc7183e5356c690dc63b4ba817470501a@chromium.org>

coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec