drm/i915/dp: change eDP default scaling mode to respect aspect ratio

Signed-off-by: Yuly Novikov <ynovikov@chromium.org>
[Jani: ripped this change separate from the scaling mode change support]
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Tested-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>