drm/amd/display: dc/clk_mgr: make function static

[Why]
linux-next kernel test robot reported the following problem:
warning: no previous prototype for 'dce60_get_dp_ref_freq_khz' [-Wmissing-prototypes]

[How]
mark dce60_get_dp_ref_freq_khz() as static

Fixes: 3ecb3b7 "drm/amd/display: dc/clk_mgr: add support for SI parts (v2)"
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Mauro Rossi <issor.oruam@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/dc/clk_mgr/dce60/dce60_clk_mgr.c

@@ -80,7 +80,7 @@ static const struct state_dependent_clocks dce60_max_clks_by_state[] = {
 /* ClocksStatePerformance */
 { .display_clk_khz = 600000, .pixel_clk_khz = 400000 } };
 
-int dce60_get_dp_ref_freq_khz(struct clk_mgr *clk_mgr_base)
+static int dce60_get_dp_ref_freq_khz(struct clk_mgr *clk_mgr_base)
 {
 	struct clk_mgr_internal *clk_mgr = TO_CLK_MGR_INTERNAL(clk_mgr_base);
 	int dprefclk_wdivider;