drm/amdgpu: add function parameter description in 'amdgpu_device_set_cg_state'

Fixes gcc warning:

drivers/gpu/drm/amd/amdgpu/amdgpu_device.c:1954: warning: Function
parameter or member 'state' not described in 'amdgpu_device_set_cg_state'

Fixes: e3ecdffa ("drm/amdgpu: add documentation for amdgpu_device.c")
Signed-off-by: yu kuai <yukuai3@huawei.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>