I have a Dell laptop M4800 with hybrid AMD and Intel graphics. After I upgraded to linux-4.9.10, system hangs with following message.

[drm:amdgpu_atombios_get_connector_info_from_object_table [amdgpu]] *ERROR* invalid con_obj_id 22 for device tag 0x0002
[drm:amdgpu_atombios_get_connector_info_from_object_table [amdgpu]] *ERROR* invalid con_obj_id 22 for device tag 0x0020

The only thing I could do then was force a power shutdown. However, if I disabled "Switching Hybrid Graphics" in BIOS, boot process reached a stage where SDDM failed to start (cored-dump SEGV), but I at least could switch to another VT and login.

Installing linux-next-4.10.0 didn't help either. However switching back to linux-4.8.15 made the system usable again.

Here's output from inxi -G

Graphics: Card-1: Intel 4th Gen Core Processor Integrated Graphics Controller
Card-2: Advanced Micro Devices [AMD/ATI] Venus XT [Radeon HD 8870M / R9 M270X/M370X]
Display Server: X.Org 1.19.1 driver: intel Resolution: 1920x1080@60.02hz
GLX Renderer: Mesa DRI Intel Haswell Mobile GLX Version: 3.0 Mesa 13.0.3

Any idea what the problem could be?

It doesn't look like it, but just to be sure, you were not on the build repo when broken linux-firmware was up there?

pacman -Q linux-firmware

And fails to start means...? Kernel loads but sddm does not? Possible to boot with 3 at the kernel line? (cli mode)
This bug report shows any similarity?
https://bugs.freedesktop.org/show_bug.cgi?id=98979

Yes, that bug report's attachment has the exact same error messages that I posted -- and my laptop happens to be a Dell Precision (slightly different model) same as the bug report's poster.

However, your suggestion to boot into level 3, prompted me to look at the Linux command-line in GRUB. It seems I had blacklisted some modules (microcode,radeon) way back when I had some different issues related to 4.0 kernel change. Once I removed the blacklisted modules, my laptop is now able to boot with both, linux-4.9.10 and linux-next-4.10.0. So far I haven't observed any ill effects due to no longer blacklisting the above modules. I guess at some point they became unnecessary, but didn't show any problems until the recent kernel update.

Thanks for your help!