gpu/host1x: fence: Make spinlock static

The DEFINE_SPINLOCK macro creates a global spinlock symbol that is visible
to the whole kernel. This is unintended in the code, fix it.

Fixes: 687db220 ("gpu: host1x: Add DMA fence implementation")
Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Reviewed-by: Mikko Perttunen <mperttunen@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>