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>

drivers/gpu/drm/amd/display/dc/clk_mgr/dcn31/dcn31_clk_mgr.c

@@ -615,7 +615,7 @@ static void dcn31_clk_mgr_helper_populate_bw_params(struct clk_mgr_internal *clk
 	}
 }
 
-void dcn31_set_low_power_state(struct clk_mgr *clk_mgr_base)
+static void dcn31_set_low_power_state(struct clk_mgr *clk_mgr_base)
 {
 	int display_count;
 	struct clk_mgr_internal *clk_mgr = TO_CLK_MGR_INTERNAL(clk_mgr_base);