drm/i915/guc: Move GuC priority fields in context under guc_active

Move GuC management fields in context under guc_active struct as this is
where the lock that protects theses fields lives. Also only set guc_prio
field once during context init.

v2:
 (Daniele)
  - set CONTEXT_SET_INIT

Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Signed-off-by: John Harrison <John.C.Harrison@Intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210909164744.31249-21-matthew.brost@intel.com