drm/amdgpu: fix shadow bo skip condition

Create shadow BOs only for no-compute VM context and only for dGPU.
The existing if-condition would create shadow bo for compute context
on dGPU which not what we wanted.

Signed-off-by: Nirmoy Das <nirmoy.das@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>