drm/panic: Add a private field to struct drm_scanout_buffer

This allows driver to set some private data in get_scanout_buffer(),
and re-use them in set_pixel() callback.

Signed-off-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://lore.kernel.org/r/20250624091501.257661-2-jfalempe@redhat.com


Signed-off-by: Maarten Lankhorst <dev@lankhorst.se>