drm/i915: Compartmentalize timeline_init/park/fini

Continuing on the theme of better logical organization of our code, make
the first step towards making the timeline code better isolated from wider
struct drm_i915_private.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190621070811.7006-25-tvrtko.ursulin@linux.intel.com