drm/amd/display: Block 3D Timings

- we don't yet support 3D timings in DAL3
- somehow a code path was being executed that resulted in HW programming
for a 3D timing, which caused 3D displays to show half the desktop in one
frame, and the other half of the desktop in the next frame
- blocking all 3D timings in timing generator validation until we
implement proper 3D timing support

Signed-off-by: Joshua Aberback <Joshua.Aberback@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Harry Wentland <Harry.Wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>