drm/amdgpu: disable MEC2 JT context init for Arcturus

We don't need to handle it like other asics.

Signed-off-by: John Clements <john.clements@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>