drm/amdgpu/display: Make dcn31_set_low_power_state static

It's not used outside of dcn31_clk_mgr.c.

Reported-by: kernel test robot <lkp@intel.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>