d3d12: Fix unhandled switch case warnings

Some D3D enums have invalid values (e.g. NONE) in the enum,
so default: makes sense to catch invalid values in those cases.

Reviewed-by: Erik Faye-Lund <erik.faye-lund@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7780>

src/gallium/drivers/d3d12/d3d12_screen.cpp

@@ -475,10 +475,10 @@ d3d12_get_shader_param(struct pipe_screen *pscreen,
    case PIPE_SHADER_CAP_MAX_HW_ATOMIC_COUNTER_BUFFERS:
    case PIPE_SHADER_CAP_TGSI_CONT_SUPPORTED:
       return 0; /* not implemented */
-   }
 
    /* should only get here on unhandled cases */
-   return 0;
+   default: return 0;
+   }
 }
 
 static bool