msm: kgsl: Using ion allocation instead of pmem for GEM

In GEM object allocations that are requested to be in
physical memory, pmem was used in the past.  Since pmem
has been replaced with ion, we have updated the code to
use ion to manage memory in DRM/GEM that needs to be
shared with MDP.

Change-Id: I245b863ada6778d2206baf59a98124d39ee89d4a
Signed-off-by: Michael Street <mstreet@codeaurora.org>