This patch disables display driver in u-boot which improves EDID detection
https://forum.armbian.com/topic/13651-pine-a64-no-hdmi-signal/?do=findComment&comment=99169

diff --git a/configs/Bananapi_M2_Ultra_defconfig b/configs/Bananapi_M2_Ultra_defconfig
index e3d67670..2f9874ee 100644
--- a/configs/Bananapi_M2_Ultra_defconfig
+++ b/configs/Bananapi_M2_Ultra_defconfig
@@ -20,3 +20,4 @@ CONFIG_AXP_ELDO3_VOLT=1200
 CONFIG_SCSI=y
 CONFIG_USB_EHCI_HCD=y
 CONFIG_USB_OHCI_HCD=y
+CONFIG_VIDEO_DE2=n