drm/amd/amdgpu:Fix compute ring unable to detect hang.

When compute fence did not signal, compute ring cannot detect hardware hang
because its timeout value is set to be infinite by default.

In SR-IOV and passthrough mode, if user does not declare custome timeout
value for compute ring, then use gfx ring timeout value as default. So
that when there is a ture hardware hang, compute ring can detect it.

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