drm/i915/guc: Keep request->priority for its lifetime

We also want to support preemption with GuC submission backend.
In order to do that, we need to remember the priority, like we do on
execlists path.

v2: Remove completed prio == INT_MAX optimization

Signed-off-by: Michał Winiarski <michal.winiarski@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Jeff McGee <jeff.mcgee@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20171025200020.16636-10-michal.winiarski@intel.com