drm/amd/powerplay: optimize the interface for mgpu fan boost enablement

Cover the implementation details from outside(of power). Also preparing
for expanding this to swSMU.

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