drm/i915: Update ring->emit_flush() to take a request structure

Updated the various ring->emit_flush() implementations to take a request instead
of a ringbuf/context pair.

For: VIZ-5115
Signed-off-by: John Harrison <John.C.Harrison@Intel.com>
Reviewed-by: Tomas Elf <tomas.elf@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>