drm/amdgpu: cleanup logic in amdgpu_vm_flush

Remove some of the extra checks where they don't hurt us.

Reviewed-by: Junwei Zhang <Jerry.Zhang@amd.com>
Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>