msm: kgsl: Stop using memdesc->usermem

BugLink: https://bugs.launchpad.net/bugs/1931727



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>
Signed-off-by: Amit Kushwaha <kushwaha@codeaurora.org>
(cherry picked from commit bcef603c50b4902b34c6e2d18c07950bf2f02457)
Signed-off-by: Shrirang Bagul <shrirang.bagul@canonical.com>