msm: kgsl: Stop using memdesc->usermem

It shouldn't be illegal for memory descriptors to have multiple virtual
memory mappings under certain circumstances. If that is the case
tracking a single usermem address for each memdesc no longer makes much
sense. Get rid of the memdesc->usermem member and use an atomic counter
to track mappings instead.

Change-Id: Ic0dedbad31bafcd1019ccc8e68657cb7e3c72727
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>
Signed-off-by: Harshitha Sai Neelati <hsaine@codeaurora.org>