drm/amd/display: Cleanup dce_get_dp_ref_freq_khz

[WHY&HOW]
Cleanup unnecessary code pathes as compile guards were added and removed
overtime.
Reviewed-by: Alvin Lee <alvin.lee2@amd.com>
Signed-off-by: Jerry Zuo <jerry.zuo@amd.com>
Signed-off-by: Dillon Varone <dillon.varone@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/dc/clk_mgr/dce100/dce_clk_mgr.c

@@ -131,7 +131,7 @@ int dce_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;
 	int dprefclk_src_sel;
-	int dp_ref_clk_khz = 600000;
+	int dp_ref_clk_khz;
 	int target_div;
 
 	/* ASSERT DP Reference Clock source is from DFS*/