Shadows for point lights are rendered with artifacts when graphics API is set to metal.

Steps to reproduce :

1. Create a new project
2. Remove the directional light component.
3. Create a plane and few game objects over the plane.
4. Add point lights near the game objects.
5. In the inspector, change the shadow type to 'Soft or Hard shadows'.
6. Observe that shadows are rendered with artifacts.

Reproducible on : 2018.1.0b4, 2018.1.0b3, 2018.1.0b2, 2018.1.0b1, 2018.1.0a7, 2018.1.a6 ,2018.1.0a5, 2018.1.0a4,2018.1.0a3, 2018.1.0a2

Working Fine on : 2018.1.0a1, 2017.3.0f3, 2017.3.0f2

Environment :
Occurs only on mac