drm/i915: Update mi_set_context() to take a request structure

Updated mi_set_context() to take a request structure instead of a ring and
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>