drm/i915/gt: Drop stale commentary for timeline density

We no longer allocate a contiguous set of timeline ids for all engines
upon creation, so we no longer should assume that the timelines are
densely allocated within a context. Hopefully, the set of fences used
within a workload are still dense enough for us to take advantage of
the compressed radix tree used for the syncmap.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190625233349.32371-1-chris@chris-wilson.co.uk