drm/amd/pm: move pp_force_state_enabled member to amdgpu_pm structure

As it lables an internal pm state and amdgpu_pm structure is the more
proper place than amdgpu_device structure for it.

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