drm/amd/powerplay: make power limit retrieval as asic specific

The power limit retrieval should be done per asic. Since we may
need to lookup in the pptable and that's really asic specific.

Signed-off-by: Evan Quan <evan.quan@amd.com>
Reviewed-by: Kenneth Feng <kenneth.feng@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>