drm/udl: Ensure channel is selected before using the device.

Lift configuration command from udlfb. This appears to be essential for
at least a Rextron VCUD-60, without which no URB communication occurs.

Signed-off-by: Jamie Lentin <jm@lentin.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1471904254-26545-1-git-send-email-jm@lentin.co.uk