drm-formats: save result of intersection in the first array

In the current API, we have some set operations: join, intersect and
subtract. Both join and subtract receives two DRM format arrays and save
the result in the first one.

For the intersection we have a slightly different approach, what makes
the API weird. We don't save the result in the arguments, instead we
return a new array with the result.

Modify weston_drm_format_array_intersect() in order to make it similar
to the other two set operations.

Signed-off-by: Leandro Ribeiro <leandro.ribeiro@collabora.com>
Reviewed-by: Alexandros Frantzis <alexandros.frantzis@collabora.com>