msm: kgsl: use 2mb aligment for large dma-bufs

When LPAE is enabled for the iommu, 2mb is a valid PTE size, and
ion will use 2mb chunks when possible.  Align the virtual addresses
for large dma-bufs to 2mb boundaries so that the iommu can use these
PTEs.

Change-Id: I86739bec3a89303aa65a5e068967dac18e0417d4
Signed-off-by: Jeremy Gebben <jgebben@codeaurora.org>