drm/amdgpu/pm: document power_dpm_force_performance_level

Provide documentation for power_dpm_force_performance_level
which is used to adjust things related to GPU power states.

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