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

Now that everything above has been converted to use requests, it is possible to
update init_context() to take a request pointer instead of a ring/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>