msm: kgsl: Use kzalloc to allocate scatterlists of 1 page or less

he majority of the scatterlist allocations used in KGSL are under 1
page (1 page of struct scatterlist is approximately 1024 entries
equalling 4MB of allocated buffer).  In these cases using vmalloc
for the sglist is undesirable and slow.  Add functions to check the
size of the allocation and favor kzalloc for 1 page allocations and
vmalloc for larger lists.