drm/i915/dvo: call ->mode_set callback only when the port is running

The ns2501 controller seems to need the dpll and dvo port to accept
the timing update commands. Quick testing on my x30 here seems to
indicate that other dvo controllers don't mind. So let's move the
->mode_set callback to a place where we have the port up and running
already.

Tested-by: Chris Wilson <chris@chris-wilson.co.uk>
Tested-by: Thomas Richter <thor@math.tu-berlin.de>
Cc: stable@vger.kernel.org
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>