gpu: ion: enable the kmalloc heap

There are some Ion clients who would like to use the kmalloc
heap. Enable it.

Change-Id: Ie818be9e1211d52d489ad2cb63d94e2fddfc0f01
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>
Signed-off-by: Joseph Riches <josephriches@gmail.com>