msm: kgsl: optimize sharedmem vmfault for 4KB chunks

In case only 4K pages are allocated, page in scatterlist can be found
without walking through it.

The generic vmfault handler is measured to take in average 2831ns when
using 4K chunks only, the optimized handler is lowering the average
execution time to 418ns.

Bug: 24345960
Change-Id: Ia656b5ad06ca65e8ef5c1b1d5cc867fd632a914a
Signed-off-by: Thierry Strudel <tstrudel@google.com>