DRM/i915: Restore sdvo_flags after dtd->mode->dtd Roundrtrip.

For TV and LVDS encoders intel_sdvo_set_input_timings_for_mode()
is called to pass a mode to the sdvo chip and retrieve a dtd
containing information needed to calculate the adjusted_mode which
is done by intel_sdvo_get_dtd_from_mode().
To set this adjusted_mode as input mode for the sdvo chip, a dtd is
recalculated using intel_sdvo_get_mode_from_dtd(). During this round
trip the sdvo_flags contained in the dtd obtained from the hardware
are lost.
Since these flags cannot be ignored in all cases this patch preserves
and restores them.

This regression has been introduced in

commit 6651819b
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Sun Apr 1 19:16:18 2012 +0200

    drm/i915: handle input/output sdvo timings separately in mode_set

Signed-off-by: Egbert Eich <eich@suse.de>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>