drm/i915: set interlaced bits for TRANSCONF

I'm not sure why they are needed (I didn't notice any difference in my
tests), but these bits are in our documentation and they are also set by
the Windows driver.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>