gpu: ion: Fix bug in zeroing pages in system heap

Pages are zeroed for security purposes when returned to the
ion heap.  There was a bug in this code preventing this
from happening.

Bug: 7573871
Change-Id: I79c22ee1da98f306199f3a192eaec4e81d5fd059
Signed-off-by: Rebecca Schultz Zavin <rebecca@android.com>