drm/i915/sdvo: always set positive sync polarity

This is a revert of 81a14b46.

We already set the mode polarity using the SDVO commands with struct
intel_sdvo_dtd. We have at least 3 bugs that get fixed with this patch.
The documentation, despite not clear, can also be interpreted in a way
that suggests this patch is needed.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=15766
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=42174
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=43333


Cc: stable@kernel.org
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Keith Packard <keithp@keithp.com>