drm/amd/powerplay: add function to get thermal range

Add the function to get the min and max thermal value for vega20 with
smu11 architecture.

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