pixman: Call sync_plane when importing dmabuf

Previously, `PixmanRenderer::import_dmabuf` didn't error for an fd that
isn't a dmabuf. Which caused an error later when trying to use this
ioctl.

I'm not aware of a better way to test this, so we can just add a
`sync_plane` here.

src/backend/renderer/pixman/mod.rs

@@ -743,6 +743,9 @@ impl PixmanRenderer {
             });
         }
 
+        dmabuf.sync_plane(0, DmabufSyncFlags::START | DmabufSyncFlags::READ)?;
+        dmabuf.sync_plane(0, DmabufSyncFlags::END | DmabufSyncFlags::READ)?;
+
         let image: Image<'_, '_> = unsafe {
             pixman::Image::from_raw_mut(
                 format,