drm/amd/display: Use DTBCLK as refclk instead of DPREFCLK

[Why]
Flash of corruption observed when UCLK switching after transitioning
from DTBCLK to DPREFCLK on subVP(DP) + subVP(HDMI) config
Scenario where DPREFCLK is required instead of DTBCLK is not expected

[How]
Always set the DTBCLK source as DTBCLK0
Reviewed-by: Alvin Lee <alvin.lee2@amd.com>
Acked-by: Tom Chung <chiahsuan.chung@amd.com>
Signed-off-by: Austin Zheng <austin.zheng@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/dcn32/dcn32_dccg.c

@@ -281,7 +281,8 @@ static void dccg32_set_dpstreamclk(
 	struct dcn_dccg *dccg_dcn = TO_DCN_DCCG(dccg);
 
 	/* set the dtbclk_p source */
-	dccg32_set_dtbclk_p_src(dccg, src, otg_inst);
+	/* always program refclk as DTBCLK. No use-case expected to require DPREFCLK as refclk */
+	dccg32_set_dtbclk_p_src(dccg, DTBCLK0, otg_inst);
 
 	/* enabled to select one of the DTBCLKs for pipe */
 	switch (dp_hpo_inst) {