drm/i915: Fix pipe off timeout handling for pre-gen4

The current pre-gen4 pipe off code might break out of the loop
due to the timeout, but then the fail to print the warning.

Refactor the code a bit to use wait_for() to avoid the problem,
and that we also re-check the condition after the timeout has
expired.

v2: Use wait_for()

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>