drm/amdgpu: Set doorbell range for gfx ring

If there are 2 gfx rings, the doorbell lower range of second ring
will override the first ring.

Signed-off-by: Jinzhou.Su <Jinzhou.Su@amd.com>
Reviewed-by: Huang Rui <ray.huang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>