drm/amdgpu: Support page directory update via CPU

If amdgpu.vm_update_context param is set to use CPU, then Page
Directories will be updated by CPU instead of SDMA

v2: Call amdgpu_vm_bo_wait before updating the page tables to ensure the
PD/PT BOs are free

v3: Minor changes - due to amdgpu_vm_bo_wait() prototype change, local
variable declaration order and function comments.

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