drm/nouveau: outp: Use __member_size() helper

Use __member_size() to get the size of the flex-array member at compile
time, instead of the convoluted expression `__struct_size(p) - sizeof(*p)`

Link: https://lore.kernel.org/r/aAe5o_-f5OYSTXjZ@kspp


Reviewed-by: Kees Cook <kees@kernel.org>
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Danilo Krummrich <dakr@kernel.org>