x11-drivers/xf86-video-amdgpu: Update to 19.0.0

This release brings TearFree improvments and FreeSync enablement. The latter
requires Linux 5.0 DRM.

Release notes:
https://lists.x.org/archives/xorg-announce/2019-March/002966.html

Reviewed by: zeising (x11)
Approved by: zeising (x11)