Use dmam_alloc_coherent() instead of dma_alloc_coherent(). Then we don't
need to register a custom cleanup handler.

v3: New patch
Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200716163846.174790-8-paul@crapouillou.net