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: I577e2f0b2974f70c00325cad4fa8d7270dae7854
Signed-off-by: Arun kumar <akanak@codeaurora.org>