drm/i915: properly handle interlaced bit for sdvo dtd conversion

commit 59d92bfa5f0cdf57f82f5181b0ad6af75c3fdf41 upstream.

We've simply ignored this, which isn't too great. With this, interlaced
1080i works on my HDMI screen connected through sdvo. For no apparent
reason anything else still doesn't work as it should.

While at it, give these magic numbers in the dtd proper names and
add a comment that they match with EDID detailed timings.

v2: Actually use the right bit for interlaced.

Tested-by: Peter Ross <pross@xvid.org>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>