drm/i915: Allow panel fixed modes to have differing sync polarities

Apparently some panels declare multiple modes with random
sync polarities. Seems a bit weird, but looks like Windows/GOP
doesn't care, so let follow suit and accept alternate fixed
modes regardless of their sync polarities.

v2: Don't pollute the DRM_ namespace with a define (Jani)

Closes: https://gitlab.freedesktop.org/drm/intel/-/issues/6968


Acked-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221020093938.27200-1-ville.syrjala@linux.intel.com