drm/i915/sdvo: Robustify the dtd<->drm_mode conversions

We've failed to properly clear out the flags when converting a dtd to
a drm mode. For more paranoia just memset the entire structure (and
drop the now redundant clears).

Also since

commit 135c81b8
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Sun Jul 21 21:37:09 2013 +0200

    drm/i915: clean up crtc timings computation

we don't update the crtc timings any more properly, so do that again.

v2: Remove more redundant clearing, spotted by Ville.

v3: Actually make it compile. Oops.

v4: Use a temporary structure to fill in the mode and copy it over
with drm_mode_copy. This will ensure we don't clobber the mode list or
id. Suggested by Ville.

Cc: Rodrigo Vivi <rodrigo.vivi@gmail.com>
Cc: Jesse Barnes <jbarnes@virtuousgeek.org>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reported-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
[danvet: Use the = {}; structure clearing instead of memset as
suggested by Ville.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>