malidp: Fix NULL vs IS_ERR() checking

The get_sg_table() function does not return NULL.
It returns error pointers.

Signed-off-by: Miaoqian Lin <linmq006@gmail.com>
Signed-off-by: Liviu Dudau <liviu.dudau@arm.com>
Link: https://lore.kernel.org/dri-devel/20211213072115.18098-1-linmq006@gmail.com/