drm/{i915,xe}/fb: add panic pointer member to struct intel_framebuffer

Add a panic data pointer member in struct intel_framebuffer in
preparation for breaking the artificial subclassing between
intel_framebuffer and panic structures.

Cc: Jocelyn Falempe <jfalempe@redhat.com>
Cc: Maarten Lankhorst <dev@lankhorst.se>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://lore.kernel.org/r/41f42e1de8545409274d54854aa12e0fb390e394.1756835342.git.jani.nikula@intel.com