drm/i915/sdvo: Add dot crawl property

This property is slightly unusual in that it is a boolean and so has no
GET_MAX command.

Reference:

  Bug 28636 - missing TV parameter "Dot Crawl freeze"
  https://bugs.freedesktop.org/show_bug.cgi?id=28636

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Eric Anholt <eric@anholt.net>