drm/i915: Defer context state allocation for legacy ring submission

drm/i915: Defer context state allocation for legacy ring submission

Almost from the outset for execlists, we used deferred allocation of the
logical context and rings. Then we ported the infrastructure for pinning
contexts back to legacy, and so now we are able to also implement
deferred allocation for context objects prior to first use on the legacy
submission.

v2: We still need to differentiate between legacy engines, Joonas is
fixing that but I want this first ;) (Joonas)

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170427104651.22394-1-chris@chris-wilson.co.uk
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>