drm/amd/display: writing stereo polarity register if swapped

[why]
on some displays that prefer swapped polarity we were seeing L/R images
swapped because OTG_STEREO_SYNC_OUTPUT_POLARITY would always be mapped
to 0

[how]
fix initial dal3 implementation to properly update the polarity field
according to the crtc_stereo_flags (same as
OTG_STEREO_EYE_FLAG_POLARITY)

Signed-off-by: Martin Leung <martin.leung@amd.com>
Reviewed-by: Aric Cyr <Aric.Cyr@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>