xf86-video-amdgpu: update to 18.0.0.

I'm pleased to announce the 18.0.0 release of xf86-video-amdgpu, the
Xorg driver for AMD Radeon GPUs supported by the amdgpu kernel driver.
This release supports xserver versions 1.13-1.19. It also works with
xserver 1.20 RC1, so unless something unexpected happens, it should work
with xserver 1.20 as well.

Highlights:

* New year-based versioning scheme
* Page flipping can now be used even when the HW cursor can't be used in
  some cases
* Support for 30-bit RGB colour output. Note that gamma correction is
  currently not available with 30-bit colour. Also, hardware
  acceleration is not available with 30-bit colour with xserver versions
  before 1.20.

Plus other improvements and fixes. Thanks to everybody who contributed
to this release in any way!

(wiz)