drm/i915: Protect access to driver and timeline name

Protect the access to driver and timeline name which otherwise could be
freed as dma-fence exported is signalling fences.

This prepares the code for incoming dma-fence API changes which will start
asserting these accesses are done from a RCU locked section.

Now that the safe access is handled in the dma-fence API, the external
callers such as sync_file, and our internal code paths, we can drop the
similar protection from i915_fence_get_timeline_name().

This prepares the code for incoming dma-fence API changes which will start
asserting these accesses are done from a RCU locked section.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@igalia.com>
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com> # v1
Signed-off-by: Tvrtko Ursulin <tursulin@ursulin.net>
Link: https://lore.kernel.org/r/20250610164226.10817-3-tvrtko.ursulin@igalia.com