media: coda: Add check for dcoda_iram_alloc

As the coda_iram_alloc may return NULL pointer,
it should be better to check the return value
in order to avoid NULL poineter dereference,
same as the others.

Fixes: b313bcc9 ("[media] coda: simplify IRAM setup")
Signed-off-by: Jiasheng Jiang <jiasheng@iscas.ac.cn>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>