drm/amd/powerplay: add function to get power limit for smu11 (v2)

Add smu_v11_0_get_power_limit function to get power limit vaule.

v2: update "set" to "get", it only implements "get" behavior. (Alex)

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