drm/i915/guc/ct: Group request-related variables in a sub-structure

For better isolation of the request tracking from the rest of the
CT-related data.

v2: split to separate patch, move next_fence to substructure (Michal)

Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: John Harrison <John.C.Harrison@Intel.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191217012316.13271-4-daniele.ceraolospurio@intel.com