drm/i915: Make intel_timeline_init static

Commit fb5970da

 ("drm/i915/gt: Use the kernel_context to measure the
breadcrumb size") removed the last external user for intel_timeline_init.
Mark it static.

Signed-off-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20200511102201.9275-1-mika.kuoppala@linux.intel.com