drm/{i915,xe}/panic: move framebuffer allocation where it belongs

The struct intel_framebuffer allocation naturally belongs in intel_fb.c,
not hidden inside panic implementation. Separate the panic
allocation. Drop the unnecessary struct i915_framebuffer and struct
xe_framebuffer types.

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/d29f63e0118d002fc8edd368caea7e8185418e17.1756835342.git.jani.nikula@intel.com