drm/amdgpu: Add amdgpu_sync_wait

v2: Add intr option

Helper function useful for CPU update of VM page tables. Also useful if
kernel have to synchronously wait till VM page tables are updated.

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>