The aspect ratio definitions for 64:27 and 256:135 have been added with
libdrm 2.4.95. However, Weston currently depends on libdrm 2.4.89 or
higher. Define the definitions in Weston to support libdrm older than
2.4.95.

Fixes: #332
Fixes: 6093772f ("backend-drm: Use aspect-ratio bit definitions from libdrm")
Signed-off-by: Stefan Agner <stefan@agner.ch>