After upgrading to kernel 4.19.9 in Arch Linux I have a blank screen. The
system is unusable.

My system is Asus ROG GL702ZC with RX 580:

lspci output:
description: VGA compatible controller
product: Ellesmere [Radeon RX 470/480/570/570X/580/580X]
vendor: Advanced Micro Devices, Inc. [AMD/ATI]
physical id: 0
bus info: pci@0000:0c:00.0
version: c1
width: 64 bits
clock: 33MHz
capabilities: vga_controller bus_master cap_list rom
configuration: driver=amdgpu latency=0

dmesg:
[drm] initializing kernel modesetting (POLARIS10 0x1002:0x67DF 0x1043:0x1A60
0xC1).

The culprit is
https://github.com/torvalds/linux/commit/07e3a1cfb0568b6d8d7862077029af96af6690ea

Reverting it fixed the issue. I also tried 4.20-rc7. There is no issue there.
But 4.19.11 still has it.

Setting amdgpu.dc=0 also helps with 4.19.9 - 4.19.11

Related https://bugs.archlinux.org/task/61117