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

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>