Radeon GPU not found after kernel upgraded to CentOS 7.5 - "[AMD/ATI] display unclaimed"

My system has a three monitor setup:

1. Monitor 1 connected to the onboard i915 chip
2. Monitor 2 & 3 connected to a Radeon RX 560 GPU

This configuration worked until I upgraded CentOS to the 3.10.0-862.2.3.el7.x86_64 kernel.

After the kernel upgrade only Monitor 1 (attached to the onboard i915 controller) is working. Monitor 2 & 3 aren't receiving a signal.

The output of lshw shows the Radeon display as -display UNCLAIMED.

 $ sudo lshw -c video                        
  *-display                 
       description: VGA compatible controller
       product: Xeon E3-1200 v2/3rd Gen Core processor Graphics Controller
       vendor: Intel Corporation
       physical id: 2
       bus info: pci@0000:00:02.0
       version: 09
       width: 64 bits
       clock: 33MHz
       capabilities: msi pm vga_controller bus_master cap_list rom
       configuration: driver=i915 latency=0
       resources: irq:38 memory:f7400000-f77fffff memory:d0000000-dfffffff ioport:f000(size=64)
  *-display UNCLAIMED
       description: VGA compatible controller
       product: Baffin [Radeon RX 550 640SP / RX 560]
       vendor: Advanced Micro Devices, Inc. [AMD/ATI]
       physical id: 0
       bus info: pci@0000:02:00.0
       version: cf
       width: 64 bits
       clock: 33MHz
       capabilities: pm pciexpress msi vga_controller cap_list
       configuration: latency=0
       resources: memory:e0000000-efffffff memory:f0000000-f01fffff ioport:e000(size=256) memory:f7b00000-f7b3ffff memory:f7b40000-f7b5ffff

lspci | grep VGA shows both the onboard GPU and the Radeon:

00:02.0 VGA compatible controller: Intel Corporation Xeon E3-1200 v2/3rd Gen Core processor Graphics Controller (rev 09)
02:00.0 VGA compatible controller: Advanced Micro Devices, Inc. [AMD/ATI] Baffin [Radeon RX 550 640SP / RX 560] (rev cf)

lsmod | grep radeon returned nothing so I created /etc/modules-load.d/radeon.conf to load the module at boot. The output of lsmod | grep radeon is now:

i2c_algo_bit           13413  3 i915,amdgpu,radeon
drm_kms_helper        176920  3 i915,amdgpu,radeon
ttm                    99555  2 amdgpu,radeon
drm                   397988  9 ttm,i915,drm_kms_helper,amdgpu,radeon
i2c_core               63151  8 drm,i915,i2c_i801,drm_kms_helper,i2c_algo_bit,amdgpu,radeon,videodev

The radeon module now loads during the boot process but lshw still outputs -display UNCLAIMED and the connected monitors still receive no signal.