drm/amd/display: RV stereo support

Fix moving directly from frame packed to frame sequential mode: disable
OTG_3D_STRUCTURE_EN if the stereo mode is not frame packed.

Signed-off-by: Ken Chalmers <ken.chalmers@amd.com>
Reviewed-by: Vitaly Prosyak <Vitaly.Prosyak@amd.com>
Acked-by: Harry Wentland <Harry.Wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>