drm/i915: Make timelines gt centric

Our timelines are stored inside intel_gt so we can convert the interface
to take exactly that and not i915.

At the same time re-order the params to our more typical layout and
replace the backpointer to the new containing structure.

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/20190621070811.7006-31-tvrtko.ursulin@linux.intel.com