drm/i915: Connect requests to rings at creation not submission

It makes a lot more sense (and makes future seqno -> request conversion patches
simpler) to fill in the 'ring' field of the request structure at the point of
creation rather than submission. Given that the request structure is assigned by
ring specific code and thus is locked to a ring from the start, there really is
no reason to defer this assignment.

For: VIZ-4377
Signed-off-by: John Harrison <John.C.Harrison@Intel.com>
Reviewed-by: Thomas Daniel <Thomas.Daniel@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>