drm/amd/powerplay: simplify the code around setting power limit

Use the cached max/current power limit and move the input check
to the top layer.

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