found RING0 test fail after S3 resume regression, which is
introduced by 1cfd8e237f0318e330190ac21d63c58ae6a1f66c

Because after suspend VRAM will be cleared, so driver must
unpin the GART table(resident in VRAM) during suspend so it
can be evicted to system ram and must correspondingly pin it
during resume so the GART table could be restored to VRAM.
Signed-off-by: Monk Liu <Monk.Liu@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>