gpu: host1x: Fix race in syncpt alloc/free

gpu: host1x: Fix race in syncpt alloc/free

Fix race condition between host1x_syncpt_alloc()
and host1x_syncpt_put() by using kref_put_mutex()
instead of kref_put() + manual mutex locking.

This ensures no thread can acquire the
syncpt_mutex after the refcount drops to zero
but before syncpt_release acquires it.
This prevents races where syncpoints could
be allocated while still being cleaned up
from a previous release.

Remove explicit mutex locking in syncpt_release
as kref_put_mutex() handles this atomically.

Signed-off-by: Mainak Sen <msen@nvidia.com>
Fixes: f5ba33fb9690 ("gpu: host1x: Reserve VBLANK syncpoints at initialization")
Signed-off-by: Mikko Perttunen <mperttunen@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Link: https://lore.kernel.org/r/20250707-host1x-syncpt-race-fix-v1-1-28b0776e70bc@nvidia.com