drm/amd/display: Fix warning observed in mode change on Vega

[Why]
DOUBLE_BUFFER_EN bit is getting cleared before enable blanking.
That leads to CRTC_BLANK_DATA_EN is getting updated immediately.

[How]
Get DOUBLE_BUFFER_EN bit set, the same as DCE110.
Signed-off-by: NJerry (Fangzhi) Zuo <Jerry.Zuo@amd.com>
Reviewed-by: NCharlene Liu <Charlene.Liu@amd.com>
Acked-by: NLeo Li <sunpeng.li@amd.com>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/dc/dce120/dce120_timing_generator.c

@@ -772,7 +772,7 @@ void dce120_tg_set_blank(struct timing_generator *tg,
 
 	CRTC_REG_SET(
 		CRTC0_CRTC_DOUBLE_BUFFER_CONTROL,
-		CRTC_BLANK_DATA_DOUBLE_BUFFER_EN, 0);
+		CRTC_BLANK_DATA_DOUBLE_BUFFER_EN, 1);
 
 	if (enable_blanking)
 		CRTC_REG_SET(CRTC0_CRTC_BLANK_CONTROL, CRTC_BLANK_DATA_EN, 1);