msm: kgsl: convert sg allocation to vmalloc

kmalloc allocates physically contiguous memory and
may fail for larger allocations due to fragmentation.
The large allocations are caused by the fact that the
scatterlist structure is 24 bytes and the array size
is proportional to the number of pages being mapped.

CRs-fixed: 327066
Change-Id: Ic7d15ca0399aa5c1fa01b7892cff736b3d1e4236
Signed-off-by: Jeff Boody <jboody@codeaurora.org>