ion: Do not do allocation if the size exceeding heap remaining

There is no need to do the allocation if the size exceeding heap
remaining as it is surely not be successful. The other effect
is we should not pool the pages to fullfill heap.

Change-Id: I5fc32254e428c915a1dd6ce5839bc6ba8085dfdb
Signed-off-by: Zhenhua Huang <zhenhuah@codeaurora.org>