drm/i915/guc: Restore preempt-context across S3/S4

Stolen memory is lost across S4 (hibernate) or S3-RST as it is a portion
of ordinary volatile RAM. As we allocate our rings from stolen, this may
include the rings used for our preempt context and their breadcrumb
instructions. In order to allow preemption following hibernation and
loss of stolen memory, we therefore need to repopulate the instructions
inside the lost ring upon resume. To handle both module load and resume,
we simply defer constructing the ring to first use.

Testcase: igt/drv_selftest/live_gem
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Michał Winiarski <michal.winiarski@intel.com>
Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
Reviewed-by: Michał Winiarski <michal.winiarski@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180919205432.18394-1-chris@chris-wilson.co.uk