drm/amdkfd: fix the wrong sdma instance query for renoir

Renoir only has one sdma instance, it will get failed once query the
sdma1 registers. So use switch-case instead of static register array.

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