drm/amdkfd: simplify vm_validate_pt_pd_bos

We don't need to validate and map root PD specially here,
it would be validated and mapped by amdgpu_vm_validate_pt_bos
if it is evicted.

The special case is when turning a GFX VM to a compute VM,
if vm_update_mode changed, we should make sure root PD gets
mapped. So just map root PD after updating vm->update_funcs
in amdgpu_vm_make_compute whether the vm_update_mode changed
or not.

v3:
 - Add some comments suggested by Christian.

v2:
 - Don't rename vm_validate_pt_pd_bos and make it public.

Signed-off-by: Lang Yu <Lang.Yu@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>