msm: mdss: use vmalloc for sglist allocation

In scenarios where system is running low on memory, the
sglist allocation during sg_table cloning was failing as
there was no order 4 pages available. The cloned list
is only used for iommu map/unmap SW ops and not used by HW.
So it's safe to use physically noncontinuous sglist.
Thus replacing kmalloc with vmalloc for sglist allocation.

Change-Id: I80a10377398f90756df9533a78a63f4da3fe9a43
Signed-off-by: Nirmal Abraham <nabrah@codeaurora.org>