backend-drm: Do not overwrite plane's index when creating virtual plane

Starting with commit 4cde507b "backend-drm: fix plane sorting" the plane list will have a descending order of the planes rather than ascending.

This reversed order had the side-effect of exposing the fact that we don't set-up a plane index when creating the drm_plane using the DRM virtual API. Without settting a plane index for that drm_plane we effectively overwrite the plane index which has the 0 (zero) entry.

This wasn't an issue before commit 4cde507b "backend-drm: fix plane sorting" as it seems we never picked up that plane index as being a suitable one due to the fact that those were assigned to primary planes, but after that commit, the cursor plane will be one getting the 0 (zero) plane index.

Finally, this would trip over because we attempt to place a (cursor) view on a primary plane (where it would've normally be a cursor plane) and we end up with no framebuffer ref.

This is fixed trivially by assigning a plane index, different than the ones already created by create_spirtes().

Signed-off-by: Marius Vlad marius.vlad@collabora.com