drm/i915: Update flush_all_caches() to take request structures

Updated the *_ring_flush_all_caches() functions to take requests instead of
rings or ringbuf/context pairs.

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>