drm/amdgpu: fix regression on CIK (v2)

BugLink: http://bugs.launchpad.net/bugs/1578798

This fix was written against drm-next, but when it was
backported to 4.5 as a stable fix, the driver internal
structure change was missed.  Fix that up here to avoid
a hang due to waiting for the wrong sequence number.

v2: agd: fix up commit message
Signed-off-by: Grigori Goronzy <greg@chown.ath.cx>
Cc: stable@vger.kernel.org
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>

drivers/gpu/drm/amd/amdgpu/gfx_v7_0.c

@@ -3628,7 +3628,7 @@ static void gfx_v7_0_ring_emit_vm_flush(struct amdgpu_ring *ring,
 					unsigned vm_id, uint64_t pd_addr)
 {
 	int usepfp = (ring->type == AMDGPU_RING_TYPE_GFX);
-	uint32_t seq = ring->fence_drv.sync_seq;
+	uint32_t seq = ring->fence_drv.sync_seq[ring->idx];
 	uint64_t addr = ring->fence_drv.gpu_addr;
 
 	amdgpu_ring_write(ring, PACKET3(PACKET3_WAIT_REG_MEM, 5));