drm/amdgpu: fix ring timeout issue in gfx10 sr-iov environment

commit 26c95e8 ("drm/amdgpu: set the VM pointer to NULL in
amdgpu_job_prepare") set job->vm as NULL if there is no fence. It will
cause emit switch buffer be skippen if job->vm set as NULL.

Check job rather than vm could solve this problem.

Fixes: 26c95e8 ("drm/amdgpu: set the VM pointer to NULL in amdgpu_job_prepare")
Signed-off-by: Lin.Cao <lincao12@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

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

@@ -297,7 +297,7 @@ int amdgpu_ib_schedule(struct amdgpu_ring *ring, unsigned int num_ibs,
 	amdgpu_ring_patch_cond_exec(ring, cond_exec);
 
 	ring->current_ctx = fence_ctx;
-	if (vm && ring->funcs->emit_switch_buffer)
+	if (job && ring->funcs->emit_switch_buffer)
 		amdgpu_ring_emit_switch_buffer(ring);
 
 	if (ring->funcs->emit_wave_limit &&