drm/i915: Convert 'ring_idle()' to use requests not seqnos

More seqno value to request structure conversions. Note, this change temporarily
moves the 'get_seqno()' call inside ring_idle() but this will disappear again in
a later patch when i915_seqno_passed() itself is converted.

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>