drm/i915: Convert 'last_flip_req' to be a request not a seqno

Converted 'last_flip_req' to be an actual request rather than a seqno value as
part of the on going seqno to request changes. This includes reference counting
the request being saved away to ensure it can not be retired and freed while the
overlay code is still waiting on it.

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>