simple-dmabuf-drm: support DRM_FORMAT_LINEAR for NV12 as well

This makes --import-format=NV12 testable on e.g. intel

We only set nv12_format_found to true if we found that format and at
least one understood modifier. Store modifier verbatim instead of using
a boolean flag. Last advertised and supported modifier currently wins.

The NV12 DRM_FORMAT_LINEAR image should be green in the upper left
corner and white in the lower right.

Signed-off-by: Guido Günther <agx@sigxcpu.org>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>