drm/amdgpu: fix ctx init failure for asics without gfx ring

This workaround does not affect other asics because amdgpu only need expose
one gfx sched to user for now.

Signed-off-by: Le Ma <le.ma@amd.com>
Reviewed-by: Nirmoy Das <nirmoy.das@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>