drm/i915: vblank status not valid while training display port

While the display port is in training mode, vblank interrupts don't
occur. Because we have to wait for the display port output to turn on
before starting the training sequence, enable the output in 'normal'
mode so that we can tell when a vblank has occurred, then start the
training sequence.

Signed-off-by: Keith Packard <keithp@keithp.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>