drm/nouveau: fifo: Avoid -Wflex-array-member-not-at-end warning

drm/nouveau: fifo: Avoid -Wflex-array-member-not-at-end warning

-Wflex-array-member-not-at-end was introduced in GCC-14, and we are
getting ready to enable it, globally.

Use the new TRAILING_OVERLAP() helper to fix the following warning:

drivers/gpu/drm/nouveau/nvif/fifo.c:29:42: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]

This helper creates a union between a flexible-array member (FAM)
and a set of members that would otherwise follow it. This overlays
the trailing members onto the FAM while preserving the original
memory layout.

Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Reviewed-by: Justin Stitt <justinstitt@google.com>
Acked-by: Danilo Krummrich <dakr@kernel.org>
Link: https://patch.msgid.link/aJ17oxJYcqqr3946@kspp
Signed-off-by: Kees Cook <kees@kernel.org>