drm/msm: Fix pagetables setup/teardown serialization

An atomic counter is not sufficient, as one task could still be in the
process of tearing things down while another task increments the counter
back up to one and begins setup again.  The race condition existed since
commit b145c6e6 ("drm/msm: Add support to create a local pagetable")
but got bigger in commit dbbde63c ("drm/msm: Add PRR support").

Fixes: dbbde63c ("drm/msm: Add PRR support")
Fixes: b145c6e6 ("drm/msm: Add support to create a local pagetable")
Signed-off-by: Rob Clark <robin.clark@oss.qualcomm.com>
Patchwork: https://patchwork.freedesktop.org/patch/664433/