dma-buf: heaps: Add __init to CMA and system heap module_init functions

Shrink the kernel .text a bit after successful initialization of the
heaps.

Signed-off-by: T.J. Mercier <tjmercier@google.com>
Acked-by: John Stultz <jstultz@google.com>
Signed-off-by: Sumit Semwal <sumit.semwal@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20240906000314.2368749-1-tjmercier@google.com