drm/msm: block commits during system suspend

Prevent the driver from accepting any user mode commits
while suspended. This is needed because the user space
may still be briefly active after the driver has entered
its suspend state.

CRs-Fixed: 2019307
Change-Id: Id5142914fece4bcc432ca90cc4a470216dd85a9e
Signed-off-by: Clarence Ip <cip@codeaurora.org>