drm/amdgpu: fix sdma doorbell range setting

Use the old doorbell range setting until the driver is
able to support more sdma queues.

Signed-off-by: Evan Quan <evan.quan@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>