TBT: Enter DP mode if Thunderbolt mode isn't supported
TBT: Enter DP mode if Thunderbolt mode isn't supported

This commit checks if the Thunderbolt mode is supported at TBT_START. If
the port/port-partner/cable doesn't support Thunderbolt mode, fall back
on DP mode instead.

BUG=b:162776463
BRANCH=None
TEST=Able to detect Thunderbolt docks as Display port dock on port 0.

Signed-off-by: Ayushee <5730d7ed61dc96fa04bcc2bc2e4ce32af5b2c19d@intel.com>
Change-Id: I424772c96afa9f3f11e5d4d9daa562bbbb011381
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/2335883
Reviewed-by: Abe Levkoy <2e1ad7039298f93262cf06bbb457275fb3e2ecb5@chromium.org>

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