drm/i915/gt: Make deferred context allocation explicit

Refactor the backends to handle the deferred context allocation in a
consistent manner, and allow calling it as an explicit first step in
pinning a context for the first time. This should make it easier for
backends to keep track of partially constructed contexts from
initialisation.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190809182518.20486-2-chris@chris-wilson.co.uk