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

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>
Cc: stable@vger.kernel.org
Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>