msm: kgsl: allocate the snapshot buffer from kmalloc

Having the snapshot buffer physically contiguous will make
it easier to recover from a ram dump in case the system
crashes after a hang. Also log the buffer address when
the snapshot is created so we know where to look for it.

Change-Id: I13fe603d0e9cb1118d15926ff5f8855420365c42
Signed-off-by: Jeremy Gebben <jgebben@codeaurora.org>