drm/amd/display: make get_refresh_rate() static

It's not used outside of dcn30_resource.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/dcn30/dcn30_resource.c

@@ -1899,7 +1899,7 @@ noinline bool dcn30_internal_validate_bw(
 	return out;
 }
 
-int get_refresh_rate(struct dc_state *context)
+static int get_refresh_rate(struct dc_state *context)
 {
 	int refresh_rate = 0;
 	int h_v_total = 0;