drm/i915: use mode values consistently when converting to sdvo dtd

The drm_mode->dtd conversion used the crtc timings, whereas the
dtd->drm_mod did not set these. Use the standard mode information, not
the crtc timings, in both cases to make these two functions proper
inverses of each another.

Note that this also kills the risk that we handle interlaced timings
inconsistently because the drm core uses half-frames for crtc timings,
whereas we need full frames. But interlaced support is pretty decently
broken anyway for sdvo encoders, so no big deal.

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>