drm/dp/mst: Handle invalid link bandwidth from DPCD gracefully

Don't BUG out if the link reports an invalid (or plain unknown)
bandwidth value, but report the failure and fail gracefully.

Fixes a trivial compiler warning in case the BUG is ever compiled away.

Link: http://lkml.kernel.org/p/1415785566-12758-1-git-send-email-geert@linux-m68k.org


Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Dave Airlie <airlied@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>