drm/amdgpu/soc15: Fix static checker warnings

vega10 is the only soc15 asic at the moment so these
warnings are invalid, but add a default case to silence
the warnings.

Fixes: 220ab9bd: "drm/amdgpu: soc15 enable (v3)"
Reviewed-by: Junwei Zhang <Jerry.Zhang@amd.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>