usb: typec: displayport: Export probe and remove functions

VirtualLink standard extends the DisplayPort Alt Mode by
utilizing also the USB 2 pins on the USB Type-C connector.
It uses the same messages as DisplayPort, but not the DP
SVID. At the time of writing, USB IF has not assigned a
Standard ID (SID) for VirtualLink, so the manufacturers of
VirtualLink adapters use their Vendor IDs as the SVID.

Since the SVID specific communication is exactly the same as
with DisplayPort alternate mode, there is no need to
implement separate driver for VirtualLink. We'll handle the
current VirtualLink adapters with probe drivers, and once
there is SVID assigned for it, we add it to the displayport
alt mode driver.

To support probing drivers, exporting the probe and remove
functions, and also changing the DP_HEADER helper macro to
use the SVID of the alternate mode device instead of the
DisplayPort alt mode SVID.

Suggested-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Signed-off-by: Ajay Gupta <ajayg@nvidia.com>
Signed-off-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>