drm/amdgpu: Use correct enum to set powergating state

Use enum amd_powergating_state instead of enum amd_clockgating_state.
The underlying value stays the same, so there is no functional change
in practise. This fixes a warning seen with clang:
drivers/gpu/drm/amd/amdgpu/amdgpu_device.c:1930:14: warning: implicit
      conversion from enumeration type 'enum amd_clockgating_state' to
      different enumeration type 'enum amd_powergating_state'
      [-Wenum-conversion]
                                                       AMD_CG_STATE_UNGATE);
                                                       ^~~~~~~~~~~~~~~~~~~

Signed-off-by: Stefan Agner <stefan@agner.ch>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>