drm/panfrost: Remove type name from internal structs

The two structs internal to struct panfrost_dump_object_header were
named, but sadly that is incompatible with C++, causing an error: "an
anonymous union may only have public non-static data members".

However nothing refers to struct pan_reg_hdr and struct pan_bomap_hdr
and there's no need to export these definitions, so lets drop them. This
fixes the C++ build error with the minimum change in userspace API.

Reported-by: Adrián Larumbe <adrian.larumbe@collabora.com>
Fixes: 730c2bf4 ("drm/panfrost: Add support for devcoredump")
Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Signed-off-by: Steven Price <steven.price@arm.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221017104602.142992-2-steven.price@arm.com