frontends/va: Initialize drm modifier on import.

On import we don't get a modifier so the modifier in the Gallium
handle should be set to DRM_FORMAT_MOD_INVALID instead of LINEAR.

Otherwise things like screen capture break if the driver actually
starts supporting modifiers.

Acked-by: Leo Liu <leo.liu@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7302>

src/gallium/frontends/va/surface.c

@@ -601,6 +601,7 @@ surface_from_external_memory(VADriverContextP ctx, vlVaSurface *surface,
    memset(&whandle, 0, sizeof(struct winsys_handle));
    whandle.type = WINSYS_HANDLE_TYPE_FD;
    whandle.handle = memory_attribute->buffers[index];
+   whandle.modifier = DRM_FORMAT_MOD_INVALID;
 
    // Create a resource for each plane.
    memset(resources, 0, sizeof resources);