Proprietary blob required for modesetting. Which means that Xorg and its native resolution work only with the vesa driver or fbdev from linux-libre, both with software rendering only.

dmesg

[    7.291782] [drm] radeon kernel modesetting enabled.
[    7.296222] [drm:radeon_pci_probe [radeon]] *ERROR* radeon kernel modesetting for R600 or later requires firmware installed
[    7.296271] See https://wiki.debian.org/Firmware for information about missing firmware

lspci -vvnnk

01:05.0 VGA compatible controller [0300]: Advanced Micro Devices, Inc. [AMD/ATI] RS880M [Mobility Radeon HD 4225/4250] [1002:9712] (prog-if 00 [VGA controller])
	Subsystem: Hewlett-Packard Company RS880M [Mobility Radeon HD 4225/4250] [103c:142c]
	Control: I/O+ Mem+ BusMaster+ SpecCycle- MemWINV- VGASnoop- ParErr- Stepping- SERR- FastB2B- DisINTx-
	Status: Cap+ 66MHz- UDF- FastB2B- ParErr- DEVSEL=fast >TAbort- <TAbort- <MAbort- >SERR- <PERR- INTx-
	Latency: 0, Cache Line Size: 64 bytes
	Interrupt: pin A routed to IRQ 11
	NUMA node: 0
	Region 0: Memory at 80000000 (32-bit, prefetchable) [size=256M]
	Region 1: I/O ports at 5000 [size=256]
	Region 2: Memory at 94300000 (32-bit, non-prefetchable) [size=64K]
	Region 5: Memory at 94200000 (32-bit, non-prefetchable) [size=1M]
	[virtual] Expansion ROM at 000c0000 [disabled] [size=128K]
	Capabilities: [50] Power Management version 3
		Flags: PMEClk- DSI- D1+ D2+ AuxCurrent=0mA PME(D0-,D1-,D2-,D3hot-,D3cold-)
		Status: D0 NoSoftRst- PME-Enable- DSel=0 DScale=0 PME-
	Capabilities: [a0] MSI: Enable- Count=1/1 Maskable- 64bit+
		Address: 0000000000000000  Data: 0000
	Kernel modules: radeon

Slot:	01:05.0
Class:	VGA compatible controller [0300]
Vendor:	Advanced Micro Devices [AMD] nee ATI [1002]
Device:	RS880M [Mobility Radeon HD 4200 Series] [9712]
SVendor:	Hewlett-Packard Company [103c]
SDevice:	Device [147b]

01:05.0 VGA compatible controller [0300]: Advanced Micro Devices [AMD] nee ATI RS880M [Mobility Radeon HD 4200 Series] [1002:9712]
	Subsystem: Hewlett-Packard Company Device [103c:147b]
	Kernel driver in use: radeon
	Kernel modules: radeon