drm/i915: Hold reference to intel_context over life of i915_request

Hold a reference to the intel_context over life of an i915_request.
Without this an i915_request can exist after the context has been
destroyed (e.g. request retired, context closed, but user space holds a
reference to the request from an out fence). In the case of GuC
submission + virtual engine, the engine that the request references is
also destroyed which can trigger bad pointer dref in fence ops (e.g.
i915_fence_get_driver_name). We could likely change
i915_fence_get_driver_name to avoid touching the engine but let's just
be safe and hold the intel_context reference.

v2:
 (John Harrison)
  - Update comment explaining how GuC mode and execlists mode deal with
    virtual engines differently

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/20210727002348.97202-4-matthew.brost@intel.com