drm/amdgpu: enable 4 level page table on raven (v3)

v1: enable 4 level-page table on raven
v2: add back legacy 2 level page table on raven
v3: set num_level in initial switch statement

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