As your mri suggest there is a disc bulge at L4,L5,S1 level and your symptoms are also related to it as you said it hurts while walking and is also painful while sitting. And there is anular tear also so that is the reason why there is pain.
Our disc is filled with water and that water is being protected by annular ring. So when there is a pressure on the disc the water comes out by breaking those annular ring. So there is a lot of pain when those ring breaks . Also it narrows the canal. In your case it has narrowed foraman canal so it hurts while you walk.
You should now do flexion exercise of your spine which will help to create space in canal and hence will help to reduce and then eliminate your pain. It seems in yoga you are doing more of extension exercise which is worsning your lower back.
Next Steps
Right now stop doing yoga and consult a physio to know which exercise to do and which to avoid. If you will keep on doing wrong exercise your situation will worsen.
Health Tips
Do consult a physio to fix your problem.