Enable AMD GPU support on Arch Linux

Install the necessary packages

pacman -S xf86-video-amdgpu mesa

• mesa is the DRI driver for 3D hardware acceleration. 
• xf86-video-amdgpu is the DDX driver for 2D hardware acceleration used in Xorg.

After installation enable the modules in configuration of Mkinitcpio. Open for this 

/etc/modprobe.d/modprobe.conf

Add the amdgpu driver in modules variable:

 MODULES=(amdgpu)

Build now your kernels

mkinitcpio -P

Set the kernel parameter for SI/CIK Graphicscard support. 

radeon.cik_support=0 amdgpu.cik_support=1    

radeon.si_support=0 amdgpu.si_support=1 

In addition, you can enable overclocking features with:

 amdgpu.ppfeaturemask=0xfffd7fff or amdgpu.ppfeaturemask=0xffffffff depending on the card.

  Save the changes and update grub.

update-grub
reboot