drm/i915: Move active request tracking to a vfunc

Move active request tracking to a backend vfunc rather than assuming all
backends want to do this in the manner. In the of case execlists /
ring submission the tracking is on the physical engine while with GuC
submission it is on the context.

Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: John Harrison <John.C.Harrison@Intel.com>
Signed-off-by: John Harrison <John.C.Harrison@Intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210727002348.97202-8-matthew.brost@intel.com