oak: usb_pd: avoid broken SS connection during enter DP mode.
oak: usb_pd: avoid broken SS connection during enter DP mode.

For type-c multi-function accessory (for example: apple's HDMI adapter,
or type-c dock), it has SS & DP connection. The SS connection may be
established before enter DP mode. The SS connection will be broken if
the mux is disabled during enter DP mode. So, it needs to check the mux
status and avoid to disable the mux if SS connection is already existed.

BRANCH:none
BUG=chrome-os-partner:43096
TEST=Manual on Oak rev3
1. connect apple type-C HDMI adapter to right side port(C0).
2. list usb devices in AP console:
   > lsusb
   Bus 002 Device 017: ID 05ac:100e Apple, Inc.
3. confirm that apple's device is present.

Change-Id: I47a52076de45da9f9a78d114f20c4ff6bb4aa2bc
Signed-off-by: Ben Lok <310171a828df105188e0b545d65d6370e8133283@mediatek.com>
Reviewed-on: https://chromium-review.googlesource.com/295351
Reviewed-by: Rong Chang <3a05493f7adfb36d203f663b3bc2ba30e9d6284f@chromium.org>

akappy7/ChromeOS_EC_LED_Diagnostics,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,akappy7/ChromeOS_EC_LED_Diagnostics,coreboot/chrome-ec,akappy7/ChromeOS_EC_LED_Diagnostics,akappy7/ChromeOS_EC_LED_Diagnostics,coreboot/chrome-ec,coreboot/chrome-ec,akappy7/ChromeOS_EC_LED_Diagnostics