システムログにはいくつかのACPIエラーがあり、その原因を見つける必要があると思います。
以下は、NetworkManagerとsddmが起動する前に発生するエラーです。
Aug 05 11:50:26 mymachine kernel: [drm] size 5324800
Aug 05 11:50:26 mymachine kernel: [drm] fb depth is 24
Aug 05 11:50:26 mymachine kernel: [drm] pitch is 5888
Aug 05 11:50:26 mymachine kernel: fbcon: radeondrmfb (fb0) is primary device
Aug 05 11:50:26 mymachine kernel: Console: switching to colour frame buffer device 180x56
Aug 05 11:50:26 mymachine kernel: radeon 0000:00:01.0: fb0: radeondrmfb frame buffer device
Aug 05 11:50:26 mymachine kernel: ACPI BIOS Error (bug): Could not resolve [\_SB.ALIB], AE_NOT_FOUND (20180313/psargs-330)
Aug 05 11:50:26 mymachine kernel: ACPI Error: Method parse/execution failed \_SB.PCI0.VGA.ATC0, AE_NOT_FOUND (20180313/psparse-516)
Aug 05 11:50:26 mymachine kernel: ACPI Error: Method parse/execution failed \_SB.PCI0.VGA.ATCS, AE_NOT_FOUND (20180313/psparse-516)
Aug 05 11:50:26 mymachine kernel: [drm] Initialized radeon 2.50.0 20080528 for 0000:00:01.0 on minor 0
Aug 05 11:50:26 mymachine kernel: [drm] amdgpu kernel modesetting enabled.
Aug 05 11:50:26 mymachine sddm[500]: Adding new display on vt 1 ...
Aug 05 11:50:26 mymachine sddm[500]: Loading theme configuration from ""
Aug 05 11:50:26 mymachine sddm[500]: Display server starting...
マザーボード:MSI fm2+ a68hm-e33 v2
CPU+GPU:AMD A10 7890K
オペレーティングシステム：アーキテクチャLinux