platform/chrome: cros_ec_typec: Add DP mode check

There are certain transitional situations where the dp_mode field in the
PD_CONTROL response might not be populated with the right DP pin
assignment value yet. Add a check for that to avoid sending an invalid
value to the Type C mode switch.

Signed-off-by: Prashant Malani <pmalani@chromium.org>
Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Link: https://lore.kernel.org/r/20210421042108.2002-1-pmalani@chromium.org