usb: pd: policy_engine: Disable vconn first upon disconnect

Currently vconn is getting disabled late after disabling vbus.
This is causing failure if type-C compliance TD.4.8.3(DRP Connect
Alternate Mode), as delay is disabling vconn goes above 35ms.
Fix the issue by moving vconn disable as first step upon receiving
disconnect notification. Also, making policy engine workqueue as high
priority workqueue to be able to run state machine work with lower
latency.

Change-Id: If9a979b4cc5bab3aa2ebf42397c36e5b434b50b9
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>