formats: use weston's egl config matching logic, centralize format

The GBM surface format has to match the DRM mode. Both are used in a
couple of places, so unify it so that it's only set in one place. Note
that the GBM and DRM formats are identical.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Eric Engestrom <eric.engestrom@imgtec.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>