The address of the DMA descriptor never changes. It can therefore be set
in the probe function.

v2-v3: No change
Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200716163846.174790-6-paul@crapouillou.net