drm/panic: use `///` for private items too

`///` should still be used for private items [1]. Some of the items in
this file do so already, so do it for a few other clear candidates in
the file.

Link: https://lore.kernel.org/rust-for-linux/20250416112454.2503872-1-ojeda@kernel.org/

 [1]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Reviewed-by: Christian Schrefl <chrisi.schrefl@gmail.com>
Signed-off-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://lore.kernel.org/r/20250416122106.2554208-1-ojeda@kernel.org