msm: wfd: Pick ION heap explicitly

In non-secure cases, the driver used to blindly select MM_HEAP and
the IOMMU heap to allocate from.  This worked, as when MM_HEAP
allocation failed, it would automatically fallback to IOMMU heap.
Now, when heap allocation fails, there is a scary message printed
by the ION allocators which often sets off false alarms.

This commit removes the dependancy on the implicit fallback and
explicity selects the relevant heap depending on use-case.

Change-Id: I7829d0619ef3d7d1dccf2f1c48c93e836d2a656a
Signed-off-by: Deva Ramasubramanian <dramasub@codeaurora.org>